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 serious and can be life-threatening, 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, or rarely, after some surgeries.

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, young children and adults in the United States. It more commonly causes pneumonia or ear or sinus infections. There is a vaccine to help reduce the occurrence of this infection.
  • 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. It affects mainly teenagers and young adults, and may cause local epidemics in college dormitories, boarding schools and military bases. There is a vaccine to help reduce the occurrence of this infection.
  • Haemophilus influenzae (haemophilus). Haemophilus influenzae type b (Hib) bacterium used to be the leading cause of bacterial meningitis in children. 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.
  • Listeria monocytogenes (listeria). These bacteria can be found in soft cheeses, hot dogs and luncheon meats. Fortunately, most healthy people exposed to listeria don't become ill, although pregnant women, newborns, older adults and people with weakened immune systems 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. People with weakened immune systems, due to disease or medication effect, are most vulnerable.

Viral meningitis

Each year, viruses cause a greater number of cases of meningitis than do bacteria. Viral meningitis is usually mild and often clears on its own. A group of viruses known as enteroviruses is responsible for most viral meningitis cases in the United States. These viruses tend to circulate in late summer and early fall. Viruses such as herpes simplex virus, HIV, mumps, West Nile virus and others also can cause viral meningitis.

Chronic meningitis

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 two weeks or more. Nevertheless, the signs and symptoms of chronic meningitis — headaches, fever, vomiting and mental cloudiness — are similar to those of acute meningitis.

Fungal meningitis

Fungal meningitis is relatively uncommon and causes chronic meningitis. Occasionally it can mimic acute bacterial meningitis. However, this form of meningitis isn't contagious from person to person. Cryptococcal meningitis is a common 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.

In 2012, fungal meningitis made the news because contaminated corticosteroid injections caused a multistate outbreak. Fungal meningitis cases were associated with contaminated medication injected into the spine for back or neck pain.

Other meningitis causes

Meningitis can also result from noninfectious causes, such as chemical reactions, drug allergies, some types of cancer and inflammatory diseases such as sarcoidosis.

Mar. 19, 2013