Bacteria, viruses and fungi all can cause meningitis. Identifying the source of the infection is an important part of developing a treatment plan.

Bacterial meningitis

A number of strains of bacteria can cause acute bacterial meningitis. The most common include.

• Streptococcus pneumoniae (pneumococcus). This bacterium is the most common cause of meningitis in infants and young children in the United States. It most often occurs when the bacterium 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 may also be associated with an ear infection. In these cases, it's not clear which came first — the meningitis or the ear infection — since they usually occur together.
• Neisseria meningitidis (meningococcus). This bacterium is another leading cause of bacterial meningitis. Meningococcal meningitis commonly occurs when bacteria from an upper respiratory infection enter your bloodstream. It's highly contagious and may cause local epidemics in college dormitories and boarding schools and on military bases.
• Haemophilus influenzae (haemophilus). Before the 1990s, Haemophilus influenzae type b bacterium (Hib) was the leading cause of bacterial meningitis. But new Hib vaccines given to all children in the United States as part of the routine immunization schedule have 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.
• Listeria monocytogenes (listeria). This bacterium 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. Listeria also 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, 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.

Viral meningitis

Viral meningitis is usually mild and often clears on its own in 10 days or less. A group of common viruses known as enteroviruses, which cause stomach flu, are responsible for about 90 percent of viral meningitis in the United States. These viruses tend to circulate in late summer and early fall. Viruses associated with mumps, herpes infection, West Nile virus or other diseases can also cause viral meningitis.

Chronic meningitis

Ongoing (chronic) forms of meningitis occur when slow-growing organisms invade the membranes and fluid surrounding your brain. Although acute meningitis strikes suddenly, chronic meningitis develops over four weeks or more. Nevertheless, the signs and symptoms of chronic meningitis — headaches, fever, vomiting and mental cloudiness — are similar to those of acute meningitis. This type of meningitis is rare.

fungal meningitis

Cryptococcal meningitis is a fungal form of the disease that affects people with immune deficiencies, such as AIDS. It's life-threatening if not treated with an antifungal medication.