When the Immune System Is Weakened: Viral Infections of the Brain and Spinal Cord

Table of Contents

• Key Points
• Why This Research Matters
• How This Review Was Conducted
• How the Body Defends the Brain
• Which Immune Defects Raise the Risk
• How Viruses Reach the Brain
• Diagnosing a Viral Brain Infection: The Big Picture
• Spinal Fluid Analysis and Molecular Tests
• Metagenomic Next-Generation Sequencing (mNGS)
• Antibody (Serology) Testing
• Brain Imaging and Electroencephalography (EEG)
• Brain Biopsy and Emerging Diagnostics
• Virus-by-Virus Diagnostic Features
• Herpes Simplex Virus Types 1 and 2 (HSV-1 and HSV-2)
• Varicella Zoster Virus (VZV)
• Epstein-Barr Virus (EBV)
• What This Means for Patients
• Limitations and Knowledge Gaps
• Practical Recommendations for Patients
• Frequently Asked Questions
• Source Information

Key Points

• In immunocompromised patients, viral brain and spinal cord infections are more frequent and more dangerous, and symptoms are often mild or unusual, which delays diagnosis.
• Risk maps to specific immune defects: in HIV, a CD4+ count below 200 cells per microliter raises JC virus and varicella zoster virus risk, and below 50 raises cytomegalovirus risk.
• Spinal fluid analysis is the cornerstone of diagnosis, but counts can be normal in immunocompromised patients, and early HSV PCR can be falsely negative.
• In herpes simplex encephalitis, each one-day delay in starting acyclovir was associated with worsening functional status, and mortality was six times higher in immunocompromised patients in a retrospective review.
• A positive molecular test does not always mean disease; Epstein-Barr virus was detected in up to 24 percent of patients with acute encephalitis caused by something else.

Why This Research Matters

Some viruses prefer to attack the nervous system. Scientists call this property neurotropism (the ability of a virus to infect nerve tissue). When these viruses reach the central nervous system, they can cause meningitis (inflammation of the membranes around the brain and spinal cord) or encephalitis (inflammation of the brain itself).

In people with a healthy immune system, these infections are uncommon. In people whose immune defenses are weakened, they are far more frequent and far more dangerous. This group includes patients with primary immunodeficiencies (inherited immune defects present from birth) and secondary immunodeficiencies (immune weakness caused by disease, medication, or transplant).

One problem makes these cases especially hard to manage. Immunocompromised patients often do not show the classic signs of brain infection. Their symptoms may be mild, vague, or "subclinical" (present but not obvious). That delays diagnosis and complicates treatment.

Treatment options are limited. Effective antivirals exist for only some of these viruses. In many cases, recovery depends on the patient's immune system rebuilding itself, which may be impossible or very difficult depending on the underlying diagnosis. The authors wrote this review to map out the clinical picture, the diagnostic challenges, and the current treatment approaches for these infections.

How This Review Was Conducted

This is a narrative review, not a single clinical trial. The authors gathered and summarized existing published research on immune mechanisms, viral entry into the brain, diagnostic strategies, and individual viral pathogens.

For each virus, they describe four things:

1. Epidemiology or pathogenesis (how common it is and how it causes disease)
2. Clinical presentation (how patients appear to doctors)
3. Diagnostic findings (what tests show)
4. Treatment and prevention methods

The review was published in the journal Viruses in 2026, as volume 18, article 800. It is open access, meaning anyone can read the full text free of charge under a Creative Commons license.

How the Body Defends the Brain

The brain is protected by the blood-brain barrier (BBB), a tight wall of cells that restricts the passage of immune cells into the central nervous system. This barrier keeps many threats out, but it also limits how well immune cells can patrol the brain from the inside.

Few immune cells are stationed inside the brain. Antigen-presenting cells (immune cells that show pieces of invaders to other immune cells, triggering a response) such as dendritic cells and macrophages are sparse there. Despite this, the body has backup systems.

T cells and B cells located in the meningeal dura mater (the tough outer covering of the brain) can protect the brain from within. These cells continuously secrete antibodies (protective proteins) that help suppress and prevent infections. Tissue resident memory T cells (immune cells that stay in tissue after an infection and remember the threat) remain in sites of latent viral infection, including the brain, and act as ongoing surveillance.

Three types of brain cells contribute to the native antiviral response:

• Microglia — the brain's resident immune cells. They drive the type I interferon response (an early antiviral alarm system) and act mainly as responders rather than virus producers. They also present antigens to CD8+ T cells.
• Astrocytes — supporting brain cells that are the major producers of interferon-beta (IFN-β) in certain viral infections, such as varicella zoster virus.
• Neurons — nerve cells with limited antigen-presenting ability. They keep a lower baseline interferon response and rely more on autophagy (a self-cleaning process that breaks down material inside the cell) to limit viral replication without triggering their own death.

Different immunocompromised groups face different viral risks based on their specific immune defects. Most viral infections are controlled by cellular immunity — the actions of CD4+ and CD8+ T cells. The major exception is enterovirus, which requires intact humoral immunity (antibody-based immunity driven by B cells).

Which Immune Defects Raise the Risk

CD4+ T cells are central to controlling viral infections. They work in two ways: directly killing infected cells (cytolytic activity) and blocking viral replication without killing the cell (non-cytolytic activity). They also help CD8+ T cells expand, mature, and survive.

Specific examples show how important these cells are. CD4+ T cells limit polyomavirus infection in the kidney, a major viral reservoir, and they regulate the emergence of polyomavirus variants that can become more dangerous to nerves. They also help keep varicella zoster virus and cytomegalovirus (CMV) in a dormant state in neurons. CD8+ T cells control HSV-1 mainly through non-killing mechanisms.

In mouse models, profound loss of CD4+ T cells increased the severity of measles virus infection in the brain more than loss of CD8+ T cells or B cells did. This finding highlights how critical CD4+ cells are.

Defects in T cell function, particularly CD4+ cells, open the door to a wide range of viruses, including the Herpesviridae family and the Polyomaviridae family, plus measles virus. Groups at risk include:

• People with primary T cell immunodeficiency, such as severe combined immunodeficiency (SCID)
• People living with HIV (PLWH), where risk and prognosis are stratified by CD4+ count
• Patients receiving T cell-depleting therapies, including hematopoietic stem cell transplant (HCT) recipients, solid organ transplant (SOT) recipients, patients with graft-versus-host disease (GvHD, where donor immune cells attack the recipient's body), and patients with hematologic malignancy (blood cancers)

One study of HCT patients found a post-transplant risk of viral encephalitis of 1.2% (about 1 in 83 patients), particularly after exposure to conditioning medications such as alemtuzumab and OKT-3. A systematic review found mortality increased up to 37% (about 1 in 3 patients) compared with healthy populations.

Certain immunomodulating therapies trigger specific infections. Natalizumab, used to treat multiple sclerosis and Crohn's disease, can cause progressive multifocal leukoencephalopathy (PML), a severe brain disease caused by the JC virus.

On the antibody side, B cells classically produce antibodies but also help T cells expand and mature. B cell deficiency raises the risk of enteroviral infection, because antibody production is critical for controlling enterovirus. Patients with agammaglobulinemia (a near-total absence of antibodies) are known to develop persistent enteroviral meningoencephalitis.

The review summarizes specific immunodeficiencies and their linked infection risks:

• HIV/AIDS — progressive CD4+ T cell loss. Risk includes Herpesviridae, with CMV infection risk when CD4+ falls below 50 cells/µL, polyomavirus (JC virus) when CD4+ falls below 200 cells/µL, and measles virus.
• Anti-thymocyte globulin (ATG) — used as induction therapy for transplant. It causes prolonged depletion of all T cells, raising Herpesviridae risk.
• Calcineurin inhibitors (tacrolimus, cyclosporine) — used for post-transplant maintenance. They suppress T cell activation by blocking interleukin-2 (IL-2) and interferon-gamma (IFN-γ) production, raising Herpesviridae risk.
• Natalizumab — a monoclonal antibody against alpha-4 integrin that prevents T cells from crossing the blood-brain barrier, raising PML (JC virus) risk.
• X-linked agammaglobulinemia — absence of functional B cells and therefore no antibodies, raising enterovirus risk.
• Common variable immunodeficiency (CVID) — failure of B cells to mature, leading to decreased antibody secretion and enterovirus risk.
• Anti-CD20 inhibitors (rituximab) — used for hematologic malignancy and rheumatic diseases. The antibody targets CD20+ B cells, depleting circulating B cells. Risks include enterovirus, parvovirus B19, and PML (JC virus), although the correlation for PML is difficult to establish.
• SCID — combined T and B cell immunodeficiency, raising risk across a broad span of viral infections.
• HCT recipients — neutropenia and T cell and B cell depletion in the early post-transplant period, especially with conditioning agents such as alemtuzumab and OKT-3, raising risk across a broad span of viral infections.

How Viruses Reach the Brain

Neurotropic viruses enter the central nervous system in two main ways:

1. Hematogenous spread — travel through the bloodstream
2. Retrograde axonal transport — movement backward along nerve fibers

The blood-brain barrier defends against infection, but immunocompromised patients are more vulnerable. In these patients, the barrier's integrity is disrupted through direct and indirect damage to the endothelial cells (the cells lining blood vessels), release of inflammatory cytokines (signaling molecules), and invasion by immune cells.

Hematogenous spread can occur simply through viremia (virus circulating in the blood). It is best illustrated by immune cell infiltration, which enables a "Trojan horse" mechanism. In this scenario, viruses hide inside immune cells that are allowed to cross into the brain.

Viruses that target neurons enter either by traveling along peripheral nerves or by crossing the blood-brain barrier. Certain viruses — including HSV, VZV, and HHV-6 — may also reach the brain through the olfactory pathway, the route used for smell.

Diagnosing a Viral Brain Infection: The Big Picture

Cerebrospinal fluid (CSF — the fluid that surrounds the brain and spinal cord) analysis remains the cornerstone of diagnosis. Other methods support or replace it, including neuroimaging (brain scans), serology (antibody testing), and brain biopsy.

Doctors use a tiered testing approach for both immunocompetent and immunocompromised patients. The key difference: immunocompromised patients need expanded testing at earlier stages.

• First tier (all patients): rule out HSV-1, HSV-2, VZV, and enterovirus.
• First tier (immunocompromised patients, added): test for CMV, HHV-6, HHV-7, JC virus, lymphocytic choriomeningitis virus (LCMV), and West Nile virus (WNV).
• Next tier: if testing is negative or unclear, pursue next-generation sequencing.

Spinal Fluid Analysis and Molecular Tests

Doctors should collect as much CSF as possible for analysis, with a minimum of 1 mL. Larger volumes of 5–10 mL improve the sensitivity of cultures and are preferred for recovering mycobacteria and fungi. Smaller volumes are generally sufficient for viral testing.

One practical detail matters: the first tube of CSF should not be used for bacterial culture or molecular testing, because the risk of contamination is high.

Viral encephalitis may show lymphocytic pleocytosis (a high number of lymphocytes, a type of white blood cell, in the spinal fluid), elevated protein, and low-to-normal glucose. However, CSF counts may be normal in some patients, particularly those who are immunocompromised. This is an important pitfall — normal spinal fluid does not always mean no infection.

Viral culture from CSF has relatively low sensitivity, and polymerase chain reaction (PCR) testing — a method that detects viral genetic material — is preferred. The numbers make the case clearly:

• HSV is grown from CSF in only 5% of cases (about 1 in 20), while nucleic acid amplification testing (NAAT) has sensitivity and specificity above 95% for HSV encephalitis.
• For enterovirus encephalitis, culture sensitivity is better at 65–75%, but still far inferior to NAAT (above 95% sensitivity).
• Viral culture is labor-intensive and slow, with recovery times up to 8 days for enterovirus, further limiting its clinical usefulness.

CSF PCR generally has good sensitivity and specificity, but the exact performance differs for each virus. Some pathogens, particularly arboviruses (viruses spread by mosquitoes and ticks), may have cleared from the blood before symptoms of brain invasion appear. However, viremia may be prolonged in immunocompromised patients. PCR panels allow rapid detection of multiple common CNS viruses at once.

Metagenomic Next-Generation Sequencing (mNGS)

Metagenomic next-generation sequencing (mNGS) is a technology that reads all nucleic acids — DNA and RNA — in a CSF or brain tissue sample. It has allowed doctors to identify a broader range of pathogens than older methods.

The PDAID prospective multicenter study provided key evidence. Among 204 patients with meningitis or encephalitis, mNGS identified 22% of infections (13 of 58) that conventional testing missed. Seven of these results led to changes in treatment.

The technique has also identified rare pathogens, including parvovirus 4, hepatitis E virus, and astrovirus, in cases of encephalitis that were otherwise undiagnosed. These are infections doctors previously could not name.

There are real limitations, though. False positives can occur because of environmental contaminants, so results must be interpreted carefully. High CSF cell counts — above 200 cells/mL — can limit mNGS sensitivity, because the overwhelming background of human DNA drowns out the viral signal. mNGS is not considered first-line testing because of its high cost, limited availability, and potentially long turnaround time.

Antibody (Serology) Testing

Serologic testing — looking for antibodies against a virus — remains essential for pathogens that PCR detects poorly. But there are important caveats.

Antibodies may not be present in the acute phase of infection, so an early negative test does not rule out disease. Interpretation can also be complicated by antibodies left over from previous exposures. Immunocompromised patients are less likely to mount a strong antibody response in the first place.

Serology is the preferred testing method for arboviruses, including West Nile Virus, because NAAT sensitivity is below 60% in immunocompetent patients. Cross-reactivity between related virus groups can affect results, so virus-specific confirmatory neutralizing antibody testing is required.

CSF testing for IgM and IgG antibodies against VZV, EBV, and measles may be considered when suspicion remains high despite a negative PCR result.

Brain Imaging and Electroencephalography (EEG)

Magnetic resonance imaging (MRI) is the preferred imaging method over computed tomography (CT) scans, because MRI has better sensitivity and specificity. Certain imaging patterns can support a specific viral diagnosis, although the findings are often nonspecific. Importantly, negative imaging does not mean there is no disease.

Electroencephalography (EEG) — a test that records the brain's electrical activity — serves several purposes: it supports diagnosis, monitors for seizures, and provides prognostic information.

The numbers show how useful EEG can be:

• EEG abnormalities are detected in 85.8% of viral encephalitis cases, compared with abnormal CSF findings in 57.3% of cases.
• HSV encephalitis produces characteristic periodic lateralized discharges (a distinctive electrical pattern), which support the diagnosis if detected.
• Findings in other types of viral encephalitis are generally nonspecific.
• Electrographic seizures (seizure activity visible only on EEG) can be detected in about 40% of patients with encephalitis — roughly 2 in 5.
• A normal EEG is a strong predictor of survival.

Brain Biopsy and Emerging Diagnostics

Brain biopsy may become necessary when other testing remains inconclusive. Diagnostic yields are 88–90% for focal CNS lesions (abnormal areas of brain tissue), with a mortality rate of only 2–3% from the procedure itself.

Doctors should consider a biopsy in these situations:

1. Focal CNS lesions are present and other noninvasive testing is negative or gives conflicting results.
2. The patient fails to respond to empirical therapy (treatment given before a diagnosis is confirmed).
3. There is rapidly progressive neurologic deterioration without a clear diagnosis.

Brain tissue can be sent for PCR, culture, and mNGS testing.

Several emerging diagnostics are under development:

• Cell-free DNA/RNA analysis from plasma — a less invasive alternative to CSF sampling. However, sensitivity depends on viremia, which is rarely present.
• Systemic viral epitope scanning (VirScan) — a comprehensive analysis of antibodies against all known human viruses, using Phage-Immunoprecipitation Sequencing (PhIP-Seq). It is still in development and not commercially available. Complex analysis and long turnaround times limit it.
• Rapid CRISPR-based nucleic acid diagnostics — in development, and used in one instance to diagnose Japanese encephalitis virus infection.
• Cytokine and chemokine profiles — may help distinguish infectious from non-infectious causes, although they are not specific to any one virus.

Virus-by-Virus Diagnostic Features

The review compiles the spinal fluid, imaging, and test findings that distinguish each virus.

• HSV-1/HSV-2: high lymphocyte counts, high protein, normal glucose. PCR sensitivity above 95%. Imaging shows temporal lobe or inferior frontal lobe lesions. PCR can be falsely negative; repeat after 3–7 days if suspicion continues. Xanthochromia (yellowish discoloration from blood breakdown) or elevated red blood cells can be a late finding. EEG shows frontotemporal periodic lateralized discharges.
• VZV: high lymphocyte counts, high protein, normal glucose. PCR sensitivity 80–95% in immunocompromised patients. Imaging shows ischemia at the gray-white matter junction.
• EBV: high lymphocyte counts, high protein, normal glucose. PCR sensitivity above 95% in immunocompromised patients. Frequent meningeal involvement; imaging may be normal. EBV frequently reactivates (positive PCR) without causing disease. Quantifying the viral load can help distinguish true disease. CSF IgM can help with diagnosis but can be falsely positive.
• CMV: normal-to-high white cell counts with a monocytic pattern (more monocytes than lymphocytes). Protein is normal-to-high; glucose is low. PCR sensitivity 82–100%. Imaging shows periventricular lesions, ependymitis (inflammation of the brain's inner lining), and ventriculitis in under 50% of cases. Finding CMV in the blood does not prove or rule out CNS disease.
• HHV-6: normal or mildly high lymphocyte counts, normal glucose. PCR sensitivity above 95%. Imaging shows limbic hyperintensity (abnormal signal in the brain's emotional-processing region); changes are often delayed. PCR can be positive in normal people, which complicates interpretation.
• HHV-7: high lymphocyte counts, high protein, normal glucose. Imaging shows nonspecific abnormalities in the cortex, cerebellum, and hippocampus; findings are often delayed. May represent asymptomatic reactivation.
• HHV-8: normal CSF findings, normal glucose. Nonspecific imaging (very rare). CSF parameters are poorly defined because cases are so rare.
• JC virus (PML): normal, with fewer than 20 cells. Protein normal-to-high. PCR sensitivity above 95%. Imaging shows hyperintense lesions at the gray-white junction or in brainstem white matter, usually multifocal. Diagnosis relies on clinical findings, imaging, and CSF PCR.
• BK virus: normal CSF findings, normal glucose, high cell count (unspecified pattern). Imaging findings are variable and nonspecific.
• Enterovirus: high lymphocyte or neutrophil counts, normal-to-high protein, low-to-normal glucose. PCR sensitivity above 95%. Imaging is usually normal, but EV71 can cause lesions in the dorsal brainstem, cerebellum, and spinal cord.

Herpes Simplex Virus Types 1 and 2 (HSV-1 and HSV-2)

HSV uses retrograde axonal transport to establish itself in the trigeminal ganglia (a cluster of nerve cells near the face that serves as a viral hiding place). CNS infection can occur after a first infection or after reactivation from latency, using the same backward-travel mechanism to infect new parts of the brain. The virus favors the temporal lobes and orbitofrontal cortex.

Reactivation of HSV-1 from latency may be promoted by binding to glucocorticoid receptors, which means patients taking steroids are at increased risk.

How common is HSV? About 64% (roughly 2 in 3) and 13.2% (about 1 in 8) of the global population under 50 years old are infected by HSV-1 and HSV-2, respectively. A recent multicenter study identified HSV as the most common cause of encephalitis in immunocompromised patients, accounting for 18% of cases — about 1 in 6.

HSV-1 is the most common driver of herpes simplex encephalitis (HSE). HSV-2 infection can occur in immunocompromised patients and is associated with 10% of all HSE cases. HSE is the most common cause of non-epidemic acute focal viral encephalitis.

Patients typically present acutely with fever, encephalopathy (altered brain function and confusion), focal neurological deficits, and gastrointestinal symptoms. Symptoms can progress over hours or days.

Seizures are common with HSV-1 encephalitis, although they occur less frequently in immunocompromised patients. HSV-2 is more likely to present as recurrent aseptic meningitis and is less likely to cause long-term neurologic problems.

The stakes are high. Mortality is 70% (about 7 in 10 patients) if untreated, and remains high at 28% (about 1 in 4) even with treatment. Up to 27% of survivors — roughly 1 in 4 — suffer neurologic after-effects.

Diagnosis relies on CSF studies, which typically show lymphocytic pleocytosis with moderate protein elevation and normal glucose. CSF HSV PCR has high sensitivity and specificity. However, reports of false-negative HSV PCRs in the first 72 hours have led to the recommendation to repeat CSF PCR after 3–7 days if HSE is still suspected.

Brain imaging is often abnormal within 48 hours. CT and MRI can show asymmetric signal intensity in the mesial temporal lobes and extratemporal regions. Persistent imaging abnormalities can distinguish HSE from HHV-6 encephalitis, which resolves more quickly on imaging.

Treatment must not wait. Therapy should be started empirically if HSE is suspected. Acyclovir is given for 14–21 days in immunocompromised patients. In a retrospective review, mortality from HSE was six times higher in immunocompromised patients, and each one-day delay in starting acyclovir was associated with worsening functional status.

Acyclovir resistance may occur in up to 14% of immunocompromised patients — about 1 in 7. For these patients, foscarnet can be considered. Adjunctive corticosteroids for brain swelling are sometimes used. A recent randomized controlled trial of dexamethasone given for four days showed a good safety profile, although it did not improve neurologic after-effects. Treatment for HSV meningitis lasts only 10–14 days, and switching to oral medication is an option.

There is no approved vaccine. Antiviral prophylaxis with acyclovir or valacyclovir is well established for preventing mucocutaneous HSV reactivation (sores on the skin and mucous membranes) in immunocompromised patients and remains part of post-transplant regimens. However, this strategy has not been specifically studied for preventing HSE.

Varicella Zoster Virus (VZV)

Most people are exposed to VZV by 9 years of age, after which the virus remains latent in the dorsal root ganglia (nerve cell clusters next to the spinal cord). As with HSV, VZV encephalitis may occur with a first infection or after reactivation.

The burden in immunocompromised patients is substantial. Immunocompromised patients accounted for 39% of VZV encephalitis cases — nearly 2 in 5 — in a Danish cohort study. People living with HIV who have a CD4+ count below 200 cells/µL or who have immune reconstitution inflammatory syndrome (IRIS) (a paradoxical worsening of inflammation as the immune system recovers) are at highest risk of VZV-related complications, including spread to the CNS.

VZV encephalitis usually presents as fever and encephalopathy. Focal neurologic signs and seizures are less common than in HSE. A rash may or may not be present — its absence does not rule out the disease.

VZV can cause vasculopathy (damage to blood vessels), especially in immunocompromised individuals. Infection of the cerebral arteries can cause transient ischemic attack, stroke, aneurysm, and dissection. Outcomes are worse for immunocompromised patients: 24% had poor outcomes (about 1 in 4), compared with 11% (about 1 in 9) in other patients.

CSF shows lymphocytic pleocytosis with high protein and normal glucose levels. CSF PCR is quite sensitive at 80–95% in the immunocompromised population. MRI may show temporal lobe involvement similar to HSV, but VZV has characteristic ischemic lesions at the gray-white matter junction because of its tendency to damage blood vessels. Multifocal infarcts in different vascular territories may also be seen.

Delayed initiation of antivirals leads to poorer prognosis. Acyclovir is the initial treatment of choice, usually given at a higher dose than for HSE (10–15 mg/kg versus 10 mg/kg) because VZV is less sensitive to the drug. Some experts recommend switching to oral therapy after clinical improvement and fever resolution. The total duration is 7–10 days, but it is often extended to 14–21 days based on expert opinion, especially when vasculopathy is present. Adjunctive corticosteroids can be considered if vasculopathy is present.

Prevention strategies depend on the patient's underlying risk factor. CMV prophylaxis regimens in solid organ transplant recipients are often sufficient to prevent VZV, and data is insufficient to recommend long-term prophylaxis. HCT recipients are advised to receive prophylaxis with acyclovir or valacyclovir for one year after transplant, with extension based on the institution and type of transplant.

Vaccination offers partial protection. The varicella vaccine carries an efficacy of 80–85%, although effectiveness may be lower in immunosuppressed patients. The recombinant zoster vaccine shows efficacy of around 70% in preventing viral reactivation.

Epstein-Barr Virus (EBV)

EBV establishes lifelong latency in B lymphocytes (antibody-producing immune cells) and affects more than 90% of the adult population. Most people carry it silently.

A retrospective cohort analysis of 364 patients with encephalitis in China identified EBV in PCR specimens in 23.6% of cases — about 1 in 4. However, only 10.7% (about 1 in 9) were considered to have EBV-associated encephalitis. This gap illustrates a central problem: finding the virus does not always mean the virus is causing the disease.

EBV CNS disease can take many forms: meningoencephalitis, acute disseminated encephalomyelitis, acute cerebellar ataxia, neuropathies, and behavioral changes. Symptoms are usually nonspecific, with fever, headache, and encephalopathy. Mortality can be as high as 10% (about 1 in 10), but outcomes are generally favorable.

EBV encephalitis is distinct from post-transplant lymphoproliferative disorder (PTLD), a disease driven by EBV that causes massive proliferation of infected B cells, leading to severe infiltrative disease and poor outcomes. EBV can also be associated with primary CNS lymphoma in patients with AIDS or a history of transplant. PTLD and primary CNS lymphoma are beyond the scope of this review.

On diagnosis, CSF typically shows pleocytosis with elevated protein and low glucose levels. Here is the challenge: EBV PCR positivity in CSF does not consistently correlate with EBV encephalitis. EBV is detected in up to 24% of patients (about 1 in 4) with acute encephalitis caused by something else, which makes interpretation difficult.

Quantitative PCR can help sort out the possibilities:

• High EBV load with pleocytosis — suggests EBV encephalitis
• High EBV load with a low white cell count — suggests CNS lymphoma
• Low EBV load with a low white cell count — suggests post-infectious encephalitis or asymptomatic reactivation

The presence of EBV IgM in the CSF can be consistent with CNS disease. However, antibodies can also be detected because of CSF contamination after lumbar puncture (the spinal tap procedure) or transfer of antibodies across a damaged blood-brain barrier.

What This Means for Patients

The central message for patients is that immune status determines risk, presentation, and outcome. The same virus that causes mild illness in a healthy person can cause severe brain infection in someone with a weakened immune system.

Risk is not random. It maps to specific immune defects:

• HIV with a CD4+ count below 50 cells/µL raises CMV risk; below 200 cells/µL raises JC virus and VZV risk.
• In one study of stem cell transplant patients, the risk of viral encephalitis after transplant was 1.2% (about 1 in 83), and a systematic review indicated increased mortality, up to 37%, compared with healthy populations.
• Natalizumab raises the risk of PML from JC virus.
• Rituximab raises the risk of enterovirus, parvovirus B19, and PML.

Symptoms may be subtle. Immunocompromised patients are more likely to present atypically, and their CSF may look normal even when infection is present. This means clinicians must keep a lower threshold for testing, and patients should report new neurologic symptoms promptly.

Treatment delays have measurable consequences. In HSE, each one-day delay in starting acyclovir worsens functional status. In VZV, delayed antiviral treatment leads to poorer prognosis. Speed matters more than certainty in these situations.

Finally, molecular testing has a double edge. It detects more viruses than ever before, but a positive result does not always mean disease. This is especially true for EBV, which is found in up to 24% of encephalitis cases caused by other things. Doctors must combine test results with clinical findings and imaging to make the right call.

Limitations and Knowledge Gaps

The review identifies several important gaps and cautions.

First, treatment options remain limited. Effective antivirals exist for only a subset of these viruses, and in many cases recovery depends on immune reconstitution — which may be impossible or difficult depending on the patient's underlying diagnosis.

Second, molecular testing can produce findings of unclear significance. Viruses may be identified in the CNS without clear pathogenicity, meaning their presence may not be causing the illness.

Third, mNGS has practical drawbacks: high cost, limited availability, long turnaround times, vulnerability to environmental contamination, and reduced sensitivity when CSF cell counts exceed 200 cells/mL.

Fourth, serology is less reliable in immunocompromised patients, who are less likely to mount a strong antibody response. Negative antibody tests cannot be trusted to rule out infection.

Fifth, some preventive strategies have not been directly studied for the outcome that matters most. Antiviral prophylaxis with acyclovir or valacyclovir prevents skin and mucous membrane HSV outbreaks, but has not been specifically studied to prevent HSV encephalitis.

Sixth, this patient-friendly summary covers the portions of the review addressing immune mechanisms, diagnostics, HSV, VZV, and EBV. The full review also examines additional opportunistic viral pathogens mentioned in its abstract and keywords, including CMV, JC virus, and measles virus. Readers should consult the original article for those sections.

Practical Recommendations for Patients

1. Know your immune status. If you have HIV, ask about your CD4+ count. Risk thresholds matter — below 200 cells/µL raises risk for JC virus and VZV, and below 50 cells/µL raises risk for CMV.
2. Report new neurologic symptoms immediately. Fever with confusion, new weakness, seizures, unusual headaches, or personality changes deserve urgent evaluation — even if you feel only mildly unwell.
3. Do not assume a normal test means you are clear. CSF counts can be normal in immunocompromised patients, and early PCR results for HSV can be falsely negative. Repeat testing after 3–7 days may be needed.
4. Start treatment early when suspicion is high. Doctors should start acyclovir empirically when HSV encephalitis is suspected. Every day of delay worsens outcomes.
5. Ask about prophylaxis. If you have had a stem cell transplant, ask whether you should receive acyclovir or valacyclovir for one year. If you take natalizumab or rituximab, ask how your PML risk is being monitored.
6. Discuss vaccination. The varicella vaccine has 80–85% efficacy and the recombinant zoster vaccine about 70%, though protection may be lower in immunosuppressed patients. Your care team should decide on timing around your treatment.
7. Ask whether expanded testing is appropriate. Immunocompromised patients should be tested earlier and more broadly — including CMV, HHV-6, HHV-7, JC virus, LCMV, and West Nile virus — than patients with healthy immune systems.

Frequently Asked Questions

Why are brain and spinal cord viral infections more dangerous for people with weakened immune systems?

In people whose immune defenses are reduced by disease or treatment, these infections are far more frequent and far more dangerous than in people with healthy immune systems. Symptoms are often mild, vague or not obvious, which delays diagnosis and complicates treatment. Effective antivirals exist for only some of these viruses, and recovery often depends on the immune system rebuilding itself.

What symptoms should make me seek urgent help if my immune system is weakened?

Report new neurologic symptoms immediately. Fever with confusion, new weakness, seizures, unusual headaches or personality changes deserve urgent evaluation, even if you feel only mildly unwell. Immunocompromised patients often do not show the classic signs of brain infection, so symptoms may be subtle. Doctors should keep a lower threshold for testing in this group.

Can a normal spinal fluid test mean I do not have a brain infection?

No. Spinal fluid cell counts may be normal in some patients, particularly those who are immunocompromised, so normal spinal fluid does not always mean there is no infection. Early HSV PCR results can also be falsely negative, and repeating the spinal fluid PCR after 3 to 7 days may be needed if suspicion continues.

What does my CD4+ count mean for my risk of these infections?

Risk maps to specific immune defects. In people living with HIV, a CD4+ count below 200 cells per microliter raises the risk of JC virus and varicella zoster virus, and a count below 50 cells per microliter raises the risk of cytomegalovirus. Ask your care team about your CD4+ count and what it means for you.

How is a viral brain infection diagnosed, and what tests might I have?

Spinal fluid analysis remains the cornerstone of diagnosis, supported by brain imaging, antibody testing and sometimes brain biopsy. Immunocompromised patients need expanded testing at earlier stages, adding cytomegalovirus, HHV-6, HHV-7, JC virus, lymphocytic choriomeningitis virus and West Nile virus to the first-tier tests for HSV-1, HSV-2, varicella zoster virus and enterovirus.

Why does starting treatment quickly matter so much?

Speed matters more than certainty. In herpes simplex encephalitis, each one-day delay in starting acyclovir was associated with worsening functional status, and mortality was six times higher in immunocompromised patients in a retrospective review. In varicella zoster virus encephalitis, delayed antiviral treatment leads to poorer prognosis. Therapy should be started empirically when herpes simplex encephalitis is suspected.

If a test finds a virus in my spinal fluid, does that always mean it is causing my illness?

No. Molecular testing detects more viruses than ever before, but a positive result does not always mean disease. This is especially true for Epstein-Barr virus, which was detected in up to 24 percent of patients with acute encephalitis caused by something else. Doctors must combine test results with clinical findings and imaging to make the right call.

When should someone with a weakened immune system and a suspected viral brain or spinal cord infection seek a second opinion?

Seek a second opinion when the diagnosis is uncertain or testing is inconclusive. Immunocompromised patients often have mild or unusual symptoms, and spinal fluid counts can be normal even when infection is present. Early HSV PCR can be falsely negative, so repeat testing after 3–7 days may be needed. Expanded testing for CMV, HHV-6, HHV-7, JC virus, LCMV, and West Nile virus is recommended earlier in these patients. A second opinion can help confirm the diagnosis and review whether testing and treatment are complete. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

Original article title: Opportunistic CNS Viral Infections in Immunocompromised Patients.

Authors: Giuliani AAM, Law N.

Publication details: Viruses, 2026, volume 18, article 800. Published 20 July 2026. The article is open access, distributed under the terms of the Creative Commons Attribution (CC BY) license. Academic editor: Carlo Contini.

Note: This patient-friendly article is based on peer-reviewed research. It is intended for educational purposes and does not replace personalized medical advice. Patients with weakened immune systems should discuss any neurologic symptoms and prevention strategies with their own care team.