Meningitis is a rare but deadly disease. It is usually caused by a viral infection, although it may also be caused by bacterial or fungal infections.

Certain populations have an increased risk of developing meningitis, including:

• Older adults
  
  
• Children under age 5
  
  
• Young adults
  
  
• People who live in community settings (e.g., college dormitory)
  
  
• Pregnant women
  
  
• People with compromised immune systems
  
  
• Travelers to areas with meningitis outbreaks. These include parts of sub-Saharan Africa and parts of the Middle East during religious pilgrimages.

Viral (aseptic) meningitis is the most common form of the disease. 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.  An enterovirus or chickenpox virus may reach the bloodstream and cause viral meningitis in one patient. Another person may contract that original enterovirus or chickenpox virus, but it may not develop into meningitis in the second person.

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

• Enteroviruses. The most common cause of viral meningitis, they are responsible for about 90 percent of all cases, according to the U.S. Centers for Disease Control and Prevention. Enteroviruses (e.g., coxsackievirus, 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 summer and fall months. People usually develop symptoms between three and seven days after they are infected and may be contagious beginning three days after being infected and until 10 days after symptoms occur.
  
  
• 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 (Mollaret’s) meningitis infection.
  
  
• Human immunodeficiency virus (HIV). Patients with HIV may occasionally develop meningitis or meningoencephalitis (meningitis accompanied by encephalitis). 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 child vaccinations for these diseases.

Other common viruses may occasionally cause viral meningitis (e.g., arboviruses that cause Lyme disease, chickenpox, mononucleosis).

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. It is most common in children between the ages of 1 month and 2 years and is much less common in adults. However, outbreaks of bacterial meningitis sometimes occur in groups of people who live in close quarters, such as those in college dormitories or military barracks.

Some of the more common causes of bacterial meningitis include:

• Streptococcus pneumoniae (pneumococcal). The bacteria that causes pneumonia, as well as ear and sinus infections. This is the most common cause of bacterial meningitis in adults. Although it occurs rarely, pneumococcal meningitis has a fatality rate of approximately 20 percent, according to the National Institutes of Health. Furthermore, about half of all who recover have serious long-term complications as well (e.g., deafness).
  
  
• Neisseria meningitidis (meningococcal). A relatively frequent cause of meningitis, it has increased by nearly 60 percent among adolescents and young adults since the early 1990s, according to the National Meningitis Association (NMA). Each year, about 2,500 Americans contract this form of meningitis, and 10 to 15 percent die from it, according to the NMA. In addition, about 20 percent of survivors have long-term disabilities such as brain damage, hearing loss or limb amputations. However, many people who carry the N.meningitidis 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.
  
  
• Haemophilus influenzae b (Hib). Formerly the most common cause of 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. People who work with animals, who are diagnosed with kidney failure or who take corticosteroids (which suppress the immune system) are at increased risk for this form of meningitis.
  
  
• 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. However, this is rare and usually occurs in patients whose immune systems are severely weakened from disease (e.g., HIV, AIDS, cancer) or in young children. In extremely rare cases, an amoeba sometimes found in fresh or standing water can cause a very rare but serious form of amebic meningitis infection.