Causes
The most common causes of meningitis are bacteria or viruses, although there
are many other causes. Rarer causes such as fungi can be seen, but often
only in people with poor immune systems. Some of the more exotic causes of meningitis,
such as parasites, are restricted to tropical countries.
Meningitis commonly occurs when bacteria or viruses make their way into
the fluid surrounding the brain. Sometimes they enter directly as a result
of an operation such as brain surgery. Sometimes they erode through the small
bones in our skull, for instance in extreme cases of severe sinusitis. Sometimes
they are carried there by our blood from an infection occurring elsewhere in
our body such as pneumonia (a lung infection). But in most cases, we don't really
understand how and why this happens.
Cryptococcus is a fungus that can cause meningitis. It usually occurs in people with weakened immune systems, such as people with AIDS, but recently there has been a more aggressive strain found on Vancouver Island's eastern coast that can infect healthy people. Tuberculosis (TB) can cause meningitis, but in the United States, it is usually seen in people who became infected with TB in their youth while growing up in other countries.
Certain medications and chemical irritants can cause in inflammation of the
brain similar to meningitis. There have also been rare reports of vaccines causing
meningitis. Viral or chemically-induced meningitis often goes away on its own.
Bacterial meningitis, on the other hand, is a very serious illness. The
different types of bacteria that can cause it aren't normally dangerous: over
half the population carries one or another of these bacteria in the back of
the nose and throat. They're commonly transmitted by coughing, sneezing, and
kissing, but can't live outside the human body for very long. When they manage
to enter the cerebrospinal fluid and begin multiplying, the bacteria cause inflammation
and other symptoms of meningitis.
There are over 50 species of bacteria that can cause meningitis. The most common causes of bacterial meningitis are Meningococcus (Neisseria meningitis), Pneumococcus (Streptococcus pneumoniae), group B Streptococcus (Streptococcus agalactiae) and E. coli (Eschericia coli). Before 1992, the bacteria hemophilus influenzae type b (Hib) was the most common cause of bacterial meningitis. Now that infants in the United States are immunized with Hib vaccine , meningitis caused by Hib is very rare.
Children under the age of two are most susceptible to meningitis. Other things
that increase the risk are:
- brain or spinal cord surgery
- head injury
- impaired or abnormal immune system
- kidney failure
- the use of corticosteroids (e.g., prednisone*)