Adults get bacterial meningitis when certain bacteria enter the bloodstream and travel to the membranes surrounding the brain and spinal cord. This can happen through respiratory droplets, contaminated food, or as a complication of another infection. The route depends on which type of bacteria is involved, and several distinct bacteria cause meningitis in adults through very different paths.
The Bacteria That Cause It
Six bacteria account for most cases of bacterial meningitis in the United States, but not all of them target adults equally. For teens and young adults, the two most common culprits are Streptococcus pneumoniae (the pneumococcus) and Neisseria meningitidis (the meningococcus). Older adults face a wider range of threats: pneumococcus remains the leading cause, but Group B Streptococcus, Haemophilus influenzae, Listeria monocytogenes, and meningococcus all play a role.
Each of these bacteria has its own way of reaching you, which is why “how you get meningitis” doesn’t have a single answer.
Spread Through Respiratory Contact
The most well-known route is person-to-person spread through respiratory and throat secretions. This is how meningococcal bacteria travel. You share saliva or spit with someone who carries the bacteria, often without either person knowing. Kissing is a classic example of close contact that can transmit it. Living with a carrier counts too, because prolonged time in shared spaces gives the bacteria more opportunity to spread.
Pneumococcal bacteria spread in a similar way, through coughs, sneezes, and close face-to-face contact. Many people carry these bacteria in their nose and throat without ever getting sick. The bacteria only cause meningitis when they manage to move beyond the respiratory tract and into the bloodstream.
It generally takes close or prolonged contact for transmission to happen. Brief encounters, like passing someone on the street, carry very little risk.
Spread Through Contaminated Food
Listeria is the exception to the respiratory rule. This bacterium spreads through food, not person-to-person contact. Deli meats, cold cuts, hot dogs, soft cheeses, and prepared deli salads like potato salad, chicken salad, and coleslaw are common sources. Fermented or dry sausages, including chorizo, pepperoni, and salami, can also carry it. Products sliced or prepared at deli counters are particularly prone to contamination.
Listeria meningitis is relatively rare compared to other types, but it disproportionately affects older adults, pregnant women, and people with weakened immune systems. Unlike most foodborne illnesses that cause stomach problems, Listeria can invade the bloodstream and reach the brain.
How Bacteria Reach the Brain
Regardless of how the bacteria first enter your body, the path to meningitis follows a consistent sequence. First, the bacteria colonize a mucosal surface, usually the lining of the nose and throat (or the gut, in the case of Listeria). From there, they cross through the mucous membrane and enter the bloodstream. Once in the blood, they multiply until they reach high enough numbers to attempt the final crossing: getting past the blood-brain barrier.
The blood-brain barrier is a tightly sealed layer of cells that normally keeps pathogens out of the central nervous system. Meningitis-causing bacteria have evolved several tricks to breach it. Some pass directly through the barrier’s cells. Others slip between the tiny gaps where cells meet. A third strategy, sometimes called the “Trojan horse” method, involves hitching a ride inside immune cells that are allowed to cross the barrier normally. Once past it, the bacteria infect the meninges, the protective membranes around the brain and spinal cord, triggering intense inflammation.
This entire process can unfold quickly. For meningococcal and pneumococcal meningitis, the incubation period ranges from 1 to 10 days, though symptoms usually appear in fewer than 4.
Risk Factors That Increase Vulnerability
Carrying bacteria in your nose or throat is common. Developing meningitis from those bacteria is not. The difference often comes down to how well your immune system can contain them before they reach the bloodstream. Several factors tip the odds in the bacteria’s favor.
A suppressed immune system is the most significant risk factor. This includes people living with HIV, those on immunosuppressive medications, and anyone with conditions like sickle cell anemia that compromise immune function. Having your spleen removed (or a spleen that doesn’t function properly) is another major vulnerability, since the spleen filters bacteria from the blood.
Cochlear implants increase the risk of pneumococcal meningitis specifically. The CDC and FDA identified this link in a 2002 study, and it remains a recognized risk factor today.
Living situations matter too. Crowded housing, college dormitories, and military barracks create the kind of close, prolonged contact that facilitates respiratory spread. Smoking is another predisposing factor. In one study of 148 adult meningitis patients at a Finnish university hospital, about 14% were smokers. Smoking damages the mucosal lining of the respiratory tract, making it easier for bacteria to cross into the bloodstream. Malnutrition and indoor air pollution round out the recognized risk factors.
What Happens After Exposure to a Case
If you’ve had close contact with someone diagnosed with meningococcal meningitis, you’re at increased risk of developing the disease yourself. Close contacts are typically offered preventive antibiotics to eliminate the bacteria before they can cause illness. This applies to household members, romantic partners, and anyone who shared respiratory secretions with the infected person.
The urgency of this prophylaxis is real. Because symptoms can appear within days of exposure, public health departments move quickly to identify and treat close contacts. If you learn that someone you’ve been in close contact with has been diagnosed with bacterial meningitis, reaching out to your local health department or a healthcare provider promptly is the practical next step.
Vaccines That Prevent It
Vaccines are the most effective way to reduce your risk of bacterial meningitis. Two categories of meningococcal vaccines exist for adults: MenACWY, which protects against four strains, and MenB, which targets a fifth strain. The number of doses you need (one to three) depends on your specific risk factors, such as whether you have a compromised immune system or live in a high-risk setting.
Pneumococcal vaccines protect against the most common cause of adult meningitis. Several formulations are available, and the recommended schedule varies based on age and medical history.
These vaccines don’t cover every bacterium that causes meningitis. There’s no vaccine for Listeria, for instance, which is why food safety practices (avoiding high-risk deli products or heating them thoroughly) remain an important preventive measure for older adults and immunocompromised individuals.