Meningitis - Causes

Meningitis may be caused by bacteria, viruses, fungi, head injuries, infections in other parts of the body, and other factors. The type of meningitis a person is most likely to contract depends on his or her age, habits, living environment, and health status.

Bacteria are not the most common cause of meningitis. But they produce the most serious and most life-threatening forms of the disease. The most common kinds of meningitis in newborns are those caused by streptococci (pronounced STREP-tuh-KOK-see) bacteria. These bacteria pass from the mother to the child through the blood system they share before birth. The highest incidence (rate) of meningitis occurs in babies under the age of one month. Children up to the age of two years are also at relatively high risk for the disease.

Adults are usually infected by a different kind of bacterium. This bacterium produces a form of meningitis that has some symptoms like those of pneumonia (see pneumonia entry).

One type of bacterium causes a contagious form of meningitis. A person with this form of meningitis can pass it to others with whom he or she comes into contact. Epidemics (mass infections) of meningitis have been known to occur in crowded day-care centers and military training camps.

Meningitis is often caused by a virus. The virus is usually the same one that causes other viral infections such as mumps (see mumps entry), measles (see measles entry), chickenpox (see chickenpox entry), rabies (see rabies entry), and herpes infections (like cold sores; see herpes infections entry).

A person's general health can also increase his or her risk of developing meningitis. For example, a person with a weakened immune system is at greater risk for meningitis than one who has a healthy immune system. People with AIDS (see AIDS entry) have damaged immune systems and are less able to fight off fungal infections. These fungal infections can lead to infections of the brain and meningitis.

People who have had their spleens removed are also at higher risk for meningitis. Spleen removal may be necessary to solve some other medical problem, such as cancer of the spleen. But it may also expose the patient to greater risk for meningitis.

The most common cause of meningitis is blood-borne spread. This term means that a person already has an infection in some other part of his or her body. If that infection is not treated properly, it can become more serious and start to spread through the body by way of the bloodstream. Normally, the blood-brain barrier would keep the infectious agents out of the brain. But if huge numbers of infectious agents accumulate in the blood, some of them may get through the blood-brain barrier. They will then be able to infect the meninges and cause meningitis. Infections that occur close to the brain, such as an ear or sinus infection, pose an especially high risk for meningitis.

Meningitis can also develop because of openings in the skull. These openings can occur because of a skull fracture or a surgical procedure. These openings provide a way for infectious agents to get into the brain because the blood-brain barrier cannot prevent the infection.