Most cases of meningitis are due to a bacterial or viral infection. The infection may be primary – starting in the meninges, or secondary – spreading from an infection site located in another part of the body. Viral meningitis is the most common form of meningitis in the United States, according to National Institute of Neurological Disorders and Stroke (NINDS). It is usually mild to moderate in severity and self-limited. Viral meningitis is frequently caused by an enterovirus or herpes virus but may also be caused by arboviruses (see Encephalitis).

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. According to NINDS, about 6,000 cases are reported annually.
• 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 meningitis that last 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. Fungal meningitis 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, and Candida species.

Amoebic meningitis is rare and can be lethal. It is caused by the free-living amoeba, Naegleria fowleri, a single-cell parasite, which can be found in warm water lakes and rivers.