What Is Meningitis?

Meningitis is a relatively rare infection that affects the delicate membranes -- called meninges (men-in'-jeez) -- that cover the brain and spinal cord. Meningitis used to occur most commonly in infants, but because a vaccine is now given to infants, this infection now occurs mainly in adults. Many forms of meningitis can be contagious among people in close contact -- in classrooms and university dorms, for example.
Outbreaks of meningitis, particularly the bacterial form, are rare in the U.S. However, since 1991 such outbreaks have been increasing for reasons not yet understood. Viral meningitis tends to be less severe, and most people recover completely. Fungal meningitis is the most rare form and generally occurs only in people with weak immune systems, such as people with AIDS.

What Causes It?
Meningitis is almost always caused by another bacterial or viral infection that began elsewhere in the body, like the ears, sinuses or upper respiratory tract.
The bacterial form of meningitis is an extremely serious illness that requires immediate medical care. If not treated quickly, it can lead to death within hours or to permanent brain damage in about 30% of people.

Bacterial meningitis is caused by any one of several bacteria. The most common forms are Hemophilus influenzae type b (common form in infants until the vaccine was introduced), Neisseria meningitidis or "meningococcus" (primarily in young adults) and Streptococcus pneumoniae or "pneumococcus" (most common form in adults). Together, these three bacteria account for about 80% of bacterial meningitis cases in the U.S.
The bacteria can spread from person to person through coughing and sneezing. If you are around someone who has bacterial meningitis, you should contact your doctor to see if anything needs to be done so you don't become infected.

In most instances, bacterial meningitis develops when bacteria get into the bloodstream from an infection in the sinuses, ears or other part of the upper respiratory tract. The bacteria then travel through the bloodstream to the brain.

Viral meningitis is more common than the bacterial form and generally - but not always -- less serious. It can be triggered by a number of viruses, including several that can cause diarrhea. The flu virus can also lead to meningitis in rare instances. More recently, the West Nile virus transmitted to humans by mosquitoes carrying the virus, has been known to cause cases of meningitis. People with viral meningitis are much less likely to have permanent brain damage after the infection resolves.

Fungal meningitis is much less common. It is usually caused by a fungus called cryptococcus, found in pigeon droppings. Fungus-related meningitis is rare in healthy people. However, someone who has an impaired immune system, as happens with AIDS, is more likely to become infected with this form of meningitis.