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Meningitis usually results from a viral infection, but the cause may also be a bacterial infection. Less commonly, a fungal infection may cause meningitis. Because bacterial infections are the most damaging, identifying the source of the infection is an important part of developing a treatment plan.

Bacterial meningitis

Acute bacterial meningitis usually occurs when bacteria enter the bloodstream and migrate to the brain and spinal cord. But it can also occur when bacteria directly invade the meninges, as a result of an ear or sinus infection or a skull fracture.

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 bacterial meningitis in infants and young children in the United States. It can also cause pneumonia and ear and sinus infections. When pneumococcal meningitis is associated with an ear infection, it's not always clear which came first — the meningitis or the ear infection — because 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. This infection is 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 (Hib) bacterium was the leading cause of bacterial meningitis. But new Hib vaccines — available as part of the routine childhood immunization schedule in the United States — 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). These bacteria can be found almost anywhere — in soil, in dust and in foods that have become contaminated. 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 tend to be more susceptible. Listeria can cross the placental barrier, and infections in late pregnancy may cause a baby to be stillborn or die shortly after birth.

Viral meningitis

Viruses cause a greater number of cases of meningitis each year than do bacteria. Viral meningitis is usually mild and often clears on its own within two weeks. A group of common viruses known as enteroviruses are responsible for about 90 percent of viral meningitis in the United States.

The most common signs and symptoms of enteroviral infections are rash, sore throat, joint aches and headache. Many older children and adults with enteroviral meningitis describe the "worst headache I've ever had." These viruses tend to circulate in late summer and early fall. Viruses associated with mumps, herpes infection, West Nile virus or other diseases also can 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
Fungal meningitis is relatively uncommon. 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.