Most cases of meningitis are due to a bacterial or viral infection, but it rarely may also be caused by certain cancers, injuries, parasites, or fungal spores in the environment. Bacterial meningitis can be life-threatening, but there are vaccines to prevent four of the five known types of bacterial meningitis. The infection may originate within the meninges (primary) or spread from an infection site located in another part of the body (secondary).
Viral meningitis, also called aseptic meningitis, is the most common form of meningitis in the United States, according to the Centers for Disease Control and Prevention. It is usually mild to moderate in severity and will usually resolve without treatment (self-limited).
- Enteroviruses are the most common cause of viral meningitis. Though enteroviruses are very common, they usually cause no symptoms or illness in most cases.
Less common causes include:
- Herpes simplex virus (HSV)
- Epstein-Barr virus (EBV)
- Varicella-zoster virus (VZV)—the cause of chickenpox and shingles
- Arboviruses—viruses spread by mosquitoes
Bacterial meningitis is generally considered a medical emergency. Acute cases can arise suddenly, with symptoms worsening within hours to a couple of days. Rapid identification and treatment is crucial. Untreated bacterial meningitis is usually fatal. While this condition can be caused by many different types (species) of bacteria, the most common causes are:
- Streptococcus pneumoniae – called pneumococcal meningitis; it is currently the most common form of bacterial meningitis in the U.S. It can also cause pneumonia, blood infections (septicemia), and ear and sinus infections. Infants under 2 years old, people with compromised immune systems, and the elderly are at greatest risk for it.
- Neisseria meningitidis – called meningococcal meningitis; it is highly contagious; people who are at risk include college students, infants, children younger than 1 year old, international travelers, and people with weakened immune systems.
- Haemophilus influenzae type b – once the most common cause of bacterial meningitis, its incidence has decreased in the U.S. because of widespread vaccination of children.
- Other causes include Listeria monocytogenes as well as Group B streptococcus and Escherichia coli, which may cause meningitis in a newborn when the mother passes the infection to her baby during delivery. Pregnant women are screened for Group B strep late in their pregnancy to determine whether there is risk of passing the infection to their babies.
Chronic meningitis is an infection that lasts for more than 4 weeks. It may be caused by microbes such as Mycobacterium tuberculosis, which causes tuberculosis, by Treponema pallidum, which causes syphilis, and by fungi.
Fungal meningitis, though rare, is most commonly seen in immune-compromised patients, such as those with AIDS, but may affect anyone.
- The most common cause is Cryptococcus neoformans (cryptococcal meningitis), thought to be contracted through inhaling dirt contaminated with bird droppings.
Other causes include:
- Coccidioides immitis
- Histoplasma capsulatum
- Candida species
Fungal meningitis is not contagious; it is not spread from person-to-person but occurs when an individual with a weakened immune system inhales spores from the environment. (For more, read the article on Fungal Infections.)
Parasitic meningitis is rare and can be lethal. One example is an infection caused by the free-living amoeba, Naegleria fowleri, a single-cell parasite, which can be found in warm water lakes and rivers. Infection occurs when the parasite enters the respiratory system through the nose of a person swimming in contaminated water. Another example is infection caused by the Schistosoma parasite. This type of infection does not occur in the U.S. but is common in other areas of the world such as Africa, South America, and South China.