Meningitis is caused by infections (bacterial, viral, fungal, or parasitic) or by non-infectious triggers like autoimmune diseases and certain medications. All of these cause inflammation of the meninges, the protective membranes surrounding the brain and spinal cord. The type of cause matters enormously: bacterial meningitis kills roughly 1 in 6 people who develop it, while viral meningitis usually resolves on its own within a couple of weeks.
How Pathogens Reach the Brain
The brain is normally shielded by a tightly sealed network of blood vessels called the blood-brain barrier. This barrier is selective about what it lets through, keeping most bacteria, viruses, and fungi out of the central nervous system. For meningitis to develop, a pathogen has to breach that barrier.
Bacteria are especially well-equipped for this. Once in the bloodstream, certain species produce proteins that latch onto the cells lining brain blood vessels, essentially picking the lock on the barrier. They also release toxins that break apart the tight junctions between those cells, creating gaps. Once bacteria slip through into the fluid surrounding the brain and spinal cord, the immune system responds aggressively. Immune cells flood in, inflammatory signals spike, and the resulting swelling and pressure are what cause the dangerous symptoms of meningitis. That immune response, not just the pathogen itself, drives much of the brain damage.
Bacterial Causes
Bacterial meningitis is the most dangerous form. Three species cause the majority of cases in otherwise healthy people. Streptococcus pneumoniae (pneumococcus) is the leading cause in adults and a common one in young children. Neisseria meningitidis (meningococcus) is particularly associated with outbreaks in teenagers, college students, and military recruits living in close quarters. Haemophilus influenzae type b was once a leading cause in children but has become far less common thanks to routine vaccination.
In newborns, the usual culprit is Group B Streptococcus, a bacterium that can be passed from mother to baby during delivery. Listeria monocytogenes, a food-borne bacterium found in unpasteurized dairy products, deli meats, and soft cheeses, is another cause that particularly affects newborns, pregnant people, and older adults with weakened immune systems.
Bacterial meningitis spreads differently depending on the species. Meningococcal meningitis passes through respiratory droplets: coughing, kissing, or sharing drinks. Listeria spreads through contaminated food rather than person-to-person contact. One in five people who survive bacterial meningitis end up with lasting complications, including hearing loss, seizures, limb weakness, vision problems, and difficulties with memory or speech. In cases complicated by blood poisoning (sepsis), scarring and even limb amputation can result.
Viral Causes
Viral meningitis is far more common than bacterial and usually far less severe. Non-polio enteroviruses are the most frequent cause in the United States, peaking in summer and early fall. These spread through contact with respiratory secretions or fecal matter, which is why handwashing is an effective preventive measure.
Other viruses that can cause meningitis include:
- Herpesviruses: Epstein-Barr virus, herpes simplex viruses, and varicella-zoster virus (the virus behind chickenpox and shingles)
- Mumps and measles viruses
- Influenza virus
- Arboviruses like West Nile virus, spread through mosquito bites
- Lymphocytic choriomeningitis virus, spread through contact with rodent urine or droppings
Most people with viral meningitis recover fully within 7 to 10 days with rest and symptom management. Antiviral medications can help in cases caused by herpes or influenza viruses, but for the majority of viral cases, no specific treatment exists beyond supportive care.
Fungal and Parasitic Causes
Fungal meningitis is rare and almost exclusively affects people with weakened immune systems, particularly those with advanced HIV/AIDS. The most common fungal cause worldwide is Cryptococcus, a fungus found in soil, bird droppings, and decaying wood. Two species, C. neoformans and C. gattii, cause most infections. C. neoformans is found throughout the world, while C. gattii is concentrated in tropical and subtropical regions, though it has been detected in the Pacific Northwest of the United States and British Columbia. People become infected by inhaling fungal spores from the environment; the infection starts in the lungs and can then spread to the brain.
Coccidioides, a fungus found in soil in the southwestern United States and parts of Central and South America, can also cause meningitis. So can Histoplasma, which thrives in areas with heavy bird or bat droppings. None of these fungal infections spread from person to person.
Parasitic meningitis is the rarest form. The most well-known cause is Naegleria fowleri, sometimes called the “brain-eating amoeba,” which lives in warm freshwater lakes and rivers. It enters the body through the nose, typically during swimming or diving, and causes a rapidly fatal infection. Other parasitic causes include certain roundworms and the larvae of tapeworms.
Non-Infectious Causes
Meningitis does not always involve an infection. Autoimmune diseases are among the most common non-infectious triggers. Conditions like lupus, rheumatoid arthritis, Sjögren syndrome, sarcoidosis, and Behçet disease can all cause the immune system to attack the meninges. Cancer that has spread to the brain, including leukemia, lymphoma, melanoma, and breast or lung cancer, can also inflame the meninges.
Certain medications can trigger meningitis as well. NSAIDs like ibuprofen and naproxen are the most commonly implicated, especially in people who already have an autoimmune condition. Some antibiotics, immune-suppressing drugs, and antiseizure medications have also been linked to drug-induced meningitis. In these cases, symptoms typically resolve once the medication is stopped. A ruptured brain cyst, whether present from birth or caused by a parasitic infection, can also spill fluid into the space around the brain and trigger inflammation.
How the Cause Is Identified
Because treatment depends entirely on the cause, identifying whether meningitis is bacterial, viral, fungal, or non-infectious is urgent. The primary diagnostic tool is a lumbar puncture (spinal tap), which collects a small sample of cerebrospinal fluid from the lower back.
The lab results paint a clear picture. In bacterial meningitis, the fluid typically contains very high white blood cell counts (usually 1,000 to 5,000 per microliter), elevated protein, and low glucose. A glucose level below 34 mg/dL or a protein level above 220 mg/dL points to a bacterial cause with more than 99% certainty. Viral meningitis shows a different pattern: moderate white blood cell counts (100 to 1,000 per microliter), normal or near-normal glucose, and only mildly elevated protein. The type of immune cells present also differs. Bacterial infections are dominated by neutrophils, a type of white blood cell that responds to bacterial threats, while viral infections show mostly lymphocytes.
Fungal meningitis can be trickier to diagnose because its fluid profile sometimes overlaps with other types. Cultures, antigen tests, and molecular testing help pin down the specific organism. For non-infectious meningitis, the diagnosis often comes after ruling out all infectious causes and identifying an underlying condition or medication.
Why the Type of Cause Matters
Bacterial meningitis is a medical emergency that requires immediate intravenous antibiotics. Every hour of delay increases the risk of death or permanent damage. Viral meningitis, by contrast, is usually managed at home with rest, fluids, and pain relievers. Fungal meningitis requires prolonged antifungal treatment that can last weeks to months, particularly in immunocompromised patients. Non-infectious meningitis is treated by addressing the underlying cause, whether that means stopping a medication, managing an autoimmune flare, or treating cancer.
Vaccines have dramatically reduced certain causes. Routine childhood vaccines protect against the leading bacterial causes (pneumococcus, meningococcus, and Haemophilus influenzae type b) as well as several viral causes, including mumps, measles, and chickenpox. The meningococcal vaccine is specifically recommended for preteens, teens heading to college, and anyone at increased risk due to immune conditions or travel to high-risk areas.