Spinal meningitis is caused by infections that inflame the protective membranes surrounding the brain and spinal cord. Bacteria, viruses, and fungi are the most common culprits, though non-infectious triggers like autoimmune diseases and certain medications can also be responsible. The type of pathogen determines how serious the illness is, how it spreads, and how it’s treated.
How Infections Reach the Spinal Membranes
The brain and spinal cord are protected by a series of barriers designed to keep pathogens out. For meningitis to develop, a germ typically has to colonize a mucous membrane (usually in the nose or throat), cross into the bloodstream, multiply there until levels are high enough, and then breach one of the barriers separating blood from the central nervous system. Bacteria can cross the blood-brain barrier itself, slip through the tissue at the choroid plexus (where spinal fluid is produced), or penetrate the membranes lining the spinal canal directly.
Physical damage to the skull can bypass this entire process. Basilar skull fractures and cerebrospinal fluid leaks create a direct path for bacteria to reach the membranes. Certain surgical procedures, including emergency brain surgery and external drainage devices, also raise the risk, particularly when operations last longer than about four and a half hours.
Bacterial Causes
Bacterial meningitis is the most dangerous form. The six leading bacterial causes in the United States are Streptococcus pneumoniae, Group B Streptococcus, E. coli, Haemophilus influenzae, Neisseria meningitidis, and Listeria monocytogenes. Which bacteria pose the greatest threat depends largely on age.
Newborns and Infants
Group B Strep, E. coli, and S. pneumoniae are the primary concerns. Newborns typically pick up Group B Strep during delivery if the mother carries it.
Children and Teens
In babies and young children, S. pneumoniae, H. influenzae, and N. meningitidis are the most common bacterial causes. By the teen years, the main threats narrow to S. pneumoniae and N. meningitidis. College campuses have reported outbreaks of meningococcal disease because the bacteria spread more easily where large groups of people live in close quarters.
Older Adults
Adults over 65 face a broader set of bacterial risks, including S. pneumoniae, Group B Strep, H. influenzae, Listeria, and N. meningitidis. Pregnant people are also at increased risk for Listeria infection, which can come from contaminated soft cheeses, deli meats, and other ready-to-eat foods.
Viral Causes
Viral meningitis is far more common than the bacterial form and usually less severe. Non-polio enteroviruses are the single most frequent cause. These are a family of viruses that spread through contact with stool, respiratory droplets, or contaminated surfaces, and they circulate most heavily in summer and early fall.
Other viruses that can trigger meningitis include herpesviruses (Epstein-Barr, herpes simplex, and the virus behind chickenpox and shingles), influenza, mumps, measles, West Nile virus, and lymphocytic choriomeningitis virus. Most cases of viral meningitis resolve on their own without specific treatment, though herpes simplex meningitis is an exception that requires antiviral medication.
Fungal Causes
Fungal meningitis is uncommon and not contagious between people. It develops when someone inhales fungal spores from the environment. The fungi most often responsible are Cryptococcus, Coccidioides, Histoplasma, Blastomyces, and Candida. Many of these live in soil and become airborne in dusty or disturbed environments like construction sites or during dust storms.
People with weakened immune systems are most vulnerable. If you live in or travel to regions where these fungi are common (the southwestern U.S. for Coccidioides, for example), wearing an N95 mask in dusty conditions and staying indoors during dust storms reduces your exposure.
Non-Infectious Causes
Meningitis doesn’t always come from an infection. Autoimmune and inflammatory conditions, including lupus, sarcoidosis, Behçet disease, and Sjögren syndrome, can cause the meninges to become inflamed on their own. Certain medications are also known triggers, particularly NSAIDs (like ibuprofen), some antibiotics, intravenous immunoglobulin, and monoclonal antibody therapies.
Cancers that spread to the meninges, whether from solid tumors or blood cancers like lymphoma and leukemia, can produce a form called neoplastic meningitis. In rare cases, vaccines for measles, mumps, rubella, varicella, yellow fever, and even the meningococcal vaccine itself have been linked to brief episodes of non-infectious meningitis.
How Bacterial Meningitis Spreads
Meningococcal bacteria, the type behind campus outbreaks, spread through respiratory and throat secretions. That means saliva, not just coughs or sneezes. Kissing is a common route. So is living in the same household or dormitory over time. The bacteria are not nearly as contagious as cold or flu viruses. Casual contact or breathing the same air in a room is generally not enough. It takes close or prolonged exposure for transmission to occur.
Viral meningitis spreads more easily, depending on the virus. Enteroviruses pass through fecal-oral contact and respiratory droplets, making hand hygiene one of the most effective preventive measures.
Who Is at Highest Risk
Children under one year old have the highest overall risk for bacterial meningitis. College students living in dorms face elevated risk compared to other teens and young adults, largely because of close living conditions. People with compromised immune systems are also especially vulnerable. This includes people living with HIV (particularly those with a low CD4 count or high viral load), people whose spleens have been removed or don’t function properly, and people with complement deficiencies, a group of immune disorders that impair the body’s ability to fight certain bacteria.
Vaccines That Prevent Bacterial Meningitis
Vaccines exist for four of the bacteria that cause meningitis, and three are widely used in the U.S.: Hib vaccines (for H. influenzae type b), pneumococcal vaccines (for S. pneumoniae), and meningococcal vaccines (for N. meningitidis).
The CDC recommends all 11- to 12-year-olds receive a MenACWY vaccine, which covers four strains of meningococcal bacteria. Because protection fades, a booster is recommended at age 16. If the first dose was given between ages 13 and 15, the booster should happen between 16 and 18. A separate vaccine, MenB, covers a fifth strain. The CDC recommends that teens and young adults discuss MenB vaccination with their healthcare provider. Those who choose it receive two doses six months apart, or a three-dose series if faster protection is needed before college.
People at increased risk due to immune conditions may need a primary series of two to four MenACWY doses starting as early as two months old, with boosters every three to five years depending on age. For MenB, high-risk individuals receive a three-dose primary series followed by boosters every two to three years.