What causes bacterial meningitis?

Pneumococcus. This bacterium is the most common cause of meningitis in adults and children. It most often occurs when the bacterium Streptococcus pneumoniae (pneumococcus) enters your bloodstream and migrates to your brain and spinal cord. You may also have this type of bacteria in your lungs, where it causes pneumonia. Pneumococcal meningitis can also result from an ear infection.

Meningococcus. This bacterium is another common cause of meningitis in children under age 5, in teens and in young adults. The incidence of meningococcal meningitis is increasing among college-aged students, especially those living in dormitories. According to the Centers for Disease Control and Prevention (CDC), the number of cases among 15- to 24-year-olds doubled between 1991 and 1997. Meningococcal meningitis commonly occurs when bacteria from an upper respiratory infection enter your bloodstream. It's highly contagious and may cause localized epidemics in college dormitories, boarding schools and on military bases. Five strains of the Neisseria meningitidis bacterium cause meningococcal meningitis: A, B, C, Y and W-135. A vaccine is available that offers some protection against all but serogroup B. The vaccine is effective for only 3 to 5 years, however. In 1999, the CDC recommended that all college students, especially those living on campus, receive the vaccine.

Haemophilus. The Haemophilus influenzae (H. influenzae) bacterium was once the leading cause of bacterial meningitis in children under age 5. But widespread use of the Hib vaccine in the United States has greatly reduced the number of cases of this type of meningitis. When it occurs, it tends to follow an upper respiratory infection, ear infection (otitis media) or sinusitis. Rates of H. influenzae meningitis and pneumococcal meningitis are highest among black, American Indian and Eskimo children, who may not have access to preventive medical care. Children in child-care settings are also at higher risk.

Listeria. Listeria monocytogenes can be found almost anywhere — in soil, in dust and in foods that have become contaminated with these bacteria. In the past, contaminated foods have included soft cheeses, hot dogs and luncheon meats. Many wild and domestic animals also carry the bacteria. Fortunately, most healthy people exposed to listeria don't become ill, although pregnant women, newborns and older adults are especially susceptible. In fact, pregnant women have a 20 times greater risk of becoming ill from this bacterium than other adults do. Listeria crosses the placental barrier, and infections in late pregnancy may cause a baby to be stillborn or die shortly after birth. In adults and children older than 2 weeks of age, exposure to the listeria bacterium can cause meningitis.

Acute bacterial meningitis also can occur when bacteria invade the meninges directly, rather than through the bloodstream. Common causes include ear or sinus infections and skull fractures.

More information on meningitis