Meningitis is usually caused by bacteria or a viral infection, although it may also be caused by some fungal infections. Children under the age of 5 years, adults between the ages of 18 and 24 years, people who live in community settings (e.g., college dormitories) patients with compromised immune systems due to chronic illnesses (e.g., diabetes, AIDS), pregnant women and people who work with animals (e.g., ranchers) have increased risk of exposure to the causes of the disease.

Viral (aseptic) meningitis is the most common type of meningitis. However, symptoms of viral meningitis tend to be very mild, with many cases going unnoticed or undiagnosed.

The viral form of the disease may be spread in several ways (e.g., poor hygiene, polluted water or sewage). However, contact with a person who has viral meningitis may not necessarily lead to the development of meningitis. Although the initial virus (e.g., chickenpox) may be infectious, it will not necessarily result in viral meningitis.  

Many viruses may cause viral meningitis. The most common include:

• Enteroviruses. The most common cause of viral meningitis. Enteroviruses (e.g., coxsackie virus, polio virus, echovirus) live in human intestines and are usually spread through biological matter (e.g., saliva, feces, mucus). However, most produce only mild symptoms (e.g., sore throat, cold) with only a very small percentage of infections resulting in the development of meningitis. Exposure to enteroviruses tends to occur in the warmer summer months.
  
  
• Herpes simplex virus (HSV). Both HSV-1 (virus that causes cold sores) and HSV-2 (virus that causes genital herpes) can cause meningitis in rare instances. HSV-2 is the leading cause of recurrent infection (Mollaret’s meningitis).
  
  
• Human immunodeficiency virus (HIV). Patients with HIV may occasionally develop meningitis or meningoencephalitis (meningitis accompanied by encephalitis, inflammation of the brain). This can be due to a suppressed immune system or direct HIV infection of the central nervous system. Most patients with this form of viral meningitis experience mild symptoms (e.g., headache, rash).
  
  
• Mumps and measles. May cause meningitis as well as encephalitis. Cases of meningitis resulting from mumps or measles are extremely rare because of the success of routine childhood vaccinations for these diseases.

Other common viruses may occasionally cause viral meningitis (e.g., arboviruses that cause Lyme disease, chickenpox, infectious mononucleosis). Recently, the spread of West Nile virus through mosquito bites has created a new source of viral meningitis in the United States.

Bacterial meningitis is far less common than viral meningitis, although it can occur very quickly and tends to be much more dangerous. It can spread from person to person and needs contact isolation. Some of the more common causes of bacterial meningitis include:

• Streptococcus pneumoniae (pneumococcal). The bacteria that causes pneumonia. This is the most common cause of bacterial meningitis in adults. Although it occurs rarely, pneumococcal meningitis has a fatality rate of about 20 percent, according to the National Institutes of Health (NIH). Furthermore, about half of all who recover have serious long-term complications (e.g., deafness, epilepsy) as well.
  
  
• Neisseria meningitides (meningococcal). A relatively frequent cause of meningitis. According to the NIH, about 10 percent of cases are fatal. However, many people who carry these bacteria never develop signs or symptoms of meningitis. These bacteria live naturally in the back of the nose, throat and upper respiratory areas of the human body. They pass between people through personal contact (e.g., touching, kissing) and by coughing or sneezing. Meningococcal bacteria cannot live outside of the body, making contagious outbreaks rare. Septicemia (blood poisoning) occasionally occurs in conjunction with meningitis when meningococcal bacteria enter the bloodstream (meningococcemia).
  
  
• Haemophilus influenzae b (Hib). Formerly the most common cause of bacterial meningitis. The recent development of a Hib vaccine has reduced instances of haemophilus meningitis substantially, with very few cases reported annually.
  
  
• Listeria monocytogenes (listeriosis). Meningitis may occur as a result of listeriosis (infection caused by eating contaminated food). This type of meningitis is most frequent in pregnant women, their fetuses, newborns and adults with weakened immune systems.
  
  
• E. coli (Escherichia coli). Bacteria that live in the colon or large intestine of healthy people. E. coli meningitis usually affects people with suppressed immune systems (e.g., cancer patients, AIDS patients). Because these bacteria may be present in the birth canal of pregnant women, some newborns (e.g., premature babies, low-birth weight babies) may have a higher risk of becoming infected during delivery.
  
  
• Bacillus tubercle (tuberculosis). A rare form of meningitis may occasionally occur when the bacterium that causes tuberculosis spreads from the lungs, attacking the meninges (membranes that cover the brain and spinal cord).
  
  
• Cochlear implants. Special hearing devices placed surgically in people with hearing loss. Rarely, bacterial meningitis may occur in people who receive cochlear implants. This may stem from a number of factors (e.g., inner ear abnormalities, surgical complications, history of ear infections).

In addition, some fungi (e.g., cryptococcus, histoplasma) may occasionally cause meningitis. This is rare, however, and usually occurs in patients whose immune systems are severely weakened from disease (e.g., HIV, AIDS, cancer) or in young children. Pools of standing water may carry amoebas that can cause a very rare but serious form of infection. Finally, traumatic injuries, drug allergies, some types of cancer and inflammatory diseases such as lupus can cause some types of meningitis.