Most cases of meningitis are due to a bacterial or viral infection. 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 National Center for Biotechnology Information. It is usually mild to moderate in severity and will usually resolve without treatment (self-limited).
Viral meningitis is most frequently caused by an enterovirus. Though enteroviruses are very common and the leading cause of viral meningitis, 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)
- Arboviruses—viruses spread by mosquitos
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. Infants under 2 years old and those with compromised immune systems are at greatest risk for it.
- Neisseria meningitidis – called meningococcal meningitis; seen in college students, infants, children, international travelers, and the immune-compromised. It is the leading cause of bacterial meningitis in children and young adults and is highly contagious.
- 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.
- Group B streptococcus, Escherichia coli, and Listeria monocytogenes are the most common causes of meningitis in the neonate and may be passed from the mother to her baby.
Chronic meningitis is an infection that lasts for more than 4 weeks. It may be caused by bacteria such as Mycobacterium tuberculosis, which causes tuberculosis, by Treponema pallidum, which causes syphilis, and by fungi.
In the U.S., there are about 4,100 cases and roughly 500 deaths caused by bacterial meningitis each year, according to the most recent numbers from the Centers for Disease Control and Prevention.
Fungal meningitis, though rare, is most commonly seen in immune-compromised patients, such as those with HIV/AIDS, but may affect anyone.
- The most common cause is Cryptococcus neoformans (cryptococcal meningitis), found in dirt and 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.
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.