Bacterial meningitis spreads mainly through respiratory droplets and throat secretions, the same route as a common cold. The bacteria that cause it are surprisingly common: roughly 10% of the general population carries meningococcal bacteria in their nose or throat at any given time without ever getting sick. Disease develops when these bacteria breach the body’s defenses, enter the bloodstream, and cross into the protective membranes surrounding the brain and spinal cord.
How the Bacteria Spread From Person to Person
The most common meningitis-causing bacteria live in the human nose and throat. They spread through coughing, sneezing, kissing, and sharing utensils, cups, or toothbrushes. This is why outbreaks tend to cluster in places where people live in close quarters: college dormitories, military barracks, and crowded housing.
Not all types spread the same way. Group B streptococcus, a leading cause of meningitis in newborns, is carried in the gut and vaginal tract. It passes from mother to baby during birth. Listeria, another bacterial cause, spreads through contaminated food rather than person-to-person contact. Deli meats, soft cheeses, pre-made salads like coleslaw or potato salad, and fermented sausages like salami and chorizo are common sources.
Most Carriers Never Get Sick
Carrying meningitis-causing bacteria is far more common than developing the disease. Carriage rates for meningococcal bacteria climb through childhood, peaking at nearly 24% in 19-year-olds before gradually declining to about 8% by age 50. The vast majority of carriers never develop meningitis. The immune system usually keeps these bacteria confined to the throat, where they cause no harm.
Disease happens when bacteria manage to invade the bloodstream and then cross the blood-brain barrier, a tightly sealed layer of cells that normally blocks pathogens from reaching the brain. Different bacteria use different strategies to get through. Some trick the barrier’s cells into absorbing them. Others exploit specific receptors on the barrier’s surface to latch on and force their way across. Pneumococcal bacteria can sometimes skip the bloodstream entirely, traveling directly from the nasal passages into the brain’s protective membranes.
Which Bacteria Cause It Depends on Your Age
The type of bacteria most likely to cause meningitis shifts across the lifespan. In newborns and young infants, group B streptococcus, E. coli, and Listeria are the primary culprits. Older infants, children, and young adults are most vulnerable to meningococcal and pneumococcal bacteria. In countries with widespread vaccination, Haemophilus influenzae type B has become rare as a cause but remains a threat where the vaccine is less available.
In older adults, pneumococcal bacteria and Listeria become the dominant threats again, along with other types of bacteria that the aging immune system struggles to fight off. Staphylococcus aureus can cause meningitis at any age and is the most common cause after a penetrating head wound.
Risk Factors That Make Infection More Likely
Living in close quarters is one of the strongest risk factors. College freshmen living in dormitories and military recruits in training facilities face elevated risk, partly because of their age (teens and young adults are prime carriers) and partly because of constant, close physical proximity to many people.
Certain medical conditions also increase susceptibility. A missing or nonfunctional spleen weakens the body’s ability to clear bacteria from the blood. Cochlear implants create a physical pathway that bacteria can follow into the brain’s protective layers. Recent head trauma or skull fractures can breach the barriers that normally keep bacteria out. Any condition that suppresses the immune system, from HIV to medications taken after an organ transplant, raises the odds.
Seasonal timing plays a role too. Cases in the U.S. consistently peak in January, February, and March, when respiratory infections circulate more freely and people spend more time indoors together.
U.S. Cases Are Rising
Bacterial meningitis remains uncommon, but the trend has shifted. In 2024, 503 confirmed and probable meningococcal disease cases were reported in the U.S., the highest number since 2013. Cases have increased sharply since 2021 and now exceed pre-pandemic levels. The reasons are still being studied, but reduced social mixing during the pandemic may have temporarily lowered population immunity to circulating strains.
Vaccines Are the Primary Defense
Vaccination is the most effective way to prevent bacterial meningitis. Two main vaccine types target different strains of meningococcal bacteria.
The MenACWY vaccine is recommended for all adolescents at age 11 or 12, with a booster at 16 because protection fades over time. If the first dose was given between ages 13 and 15, the booster should follow at 16 to 18. People at higher risk, including those without a functioning spleen, need a more extensive series starting as young as 2 months, with booster doses every 3 to 5 years depending on age.
The MenB vaccine, which covers a different bacterial strain, is handled differently. Rather than a blanket recommendation, the CDC suggests a shared decision between the patient and their doctor, typically for teens aged 16 to 18. Two doses given six months apart is the standard schedule, though students heading to college in less than six months can opt for an accelerated three-dose series for faster protection.
Separate vaccines exist for pneumococcal bacteria and Haemophilus influenzae type B, both part of routine childhood immunization schedules. These have dramatically reduced meningitis cases in children over the past several decades. No vaccine currently prevents Listeria or group B streptococcal meningitis, which is why pregnant women are screened for group B strep late in pregnancy and treated during labor if positive.