Causes

Meningitis is mostly caused by microorganisms like bacteria, viruses, parasites, and fungi. These microorganisms infect blood and the cerebrospinal fluid (CSF). Meningitis can also develop from non-infectious causes, including certain diseases like AIDS, cancer, diabetes, physical injury, or certain drugs that weaken the body’s immune system.

Bacterial Meningitis

Bacterial meningitis is usually more severe than viral meningitis. Bacterial meningitis can have serious after-effects, such as brain damage, hearing loss, limb amputation, or learning disabilities.

One of the leading causes of bacterial meningitis in children and young adults in the United States is the bacterium Neisseria meningitidis. Meningitis caused by this bacterium is known as meningococcal disease. Causes of bacterial meningitis also vary by age group:

Age Group	Causes
Newborn	Group B Streptococci, Escherichia coli, Listeria monocytogenes
Infants	Neisseria meningitidis, Haemophilus influenzae, Streptococcus pneumoniae
Children	N. meningitidis, S. pneumoniae
Adult	S. pneumoniae, N. meningitidis, Mycobacteria

Viral Meningitis

Viral meningitis is generally less severe and resolves without specific treatment.

Most viral meningitis cases in the United States, especially during the summer months, are caused by enteroviruses; however, only a small number of people with enterovirus infections actually develop meningitis.

Other viral infections that can lead to meningitis include

• Mumps
• Herpesvirus, including Epstein-Barr virus, herpes simplex viruses, varicella-zoster virus (which also causes chicken pox and shingles), measles, and influenza
• Viruses spread through mosquitoes and other insects (arboviruses)
• In rare cases LCMV (lymphocytic choriomeningitis virus), which is spread by rodents, can cause viral meningitis

Fungal Meningitis

Fungal meningitis is rare, but can be life threatening. Although anyone can get fungal meningitis, people at higher risk include those who have AIDS, leukemia, or other forms of immunodeficiency (an immune system that does not respond adequately to infections) and immunosuppression (immune system malfunction as a result of medical treatment).

The most common cause of fungal meningitis for people with immune system deficiencies, like HIV, is Cryptococcus. This disease is one of the most common causes of meningitis in Africa.

The fungus that causes thrush, Candida, can lead to meningitis in rare cases, especially in pre-mature babies with very low birth weight. 

Meningitis due to Histoplasma can happen in anybody, but people with immunodeficiencies are at a higher risk.  Histoplasma is found primarily in soil or bird/bat droppings in the Midwestern United States, although it can be seen in other places.

Soil in Southwestern United States and northern Mexico contain the fungus Coccidioides which can cause fungal meningitis.  Although anyone can get infected with coccidioidal meningitis, people at higher risk include African Americans, Filipinos, pregnant women in the third trimester, and immunocompromised persons.

Non-infectious Meningitis

Non–infectious meningitis causes include

• Cancers
• Systemic lupus erythematosus (lupus)
• Certain drugs
• Head injury
• Brain surgery