Causes

Viral infections are the most common cause of meningitis, followed by bacterial infections and, rarely, fungal and parasitic infections. Because bacterial infections can be life-threatening, identifying the cause is essential.

Bacterial meningitis

Bacteria that enter the bloodstream and travel to the brain and spinal cord cause acute bacterial meningitis. But it can also occur when bacteria directly invade the meninges. This may be caused by an ear or sinus infection, a skull fracture, or — rarely — some surgeries.

Several strains of bacteria can cause acute bacterial meningitis, most commonly:

• Streptococcus pneumoniae (pneumococcus). This bacterium is the most common cause of bacterial meningitis in infants, young children and adults in the United States. It more commonly causes pneumonia or ear or sinus infections. A vaccine can help prevent this infection.
• Neisseria meningitidis (meningococcus). This bacterium is another leading cause of bacterial meningitis. These bacteria commonly cause an upper respiratory infection but can cause meningococcal meningitis when they enter the bloodstream. This is a highly contagious infection that affects mainly teenagers and young adults. It may cause local epidemics in college dormitories, boarding schools and military bases. A vaccine can help prevent infection. Even if vaccinated, anybody who has been in close contact with a person with meningococcal meningitis should receive an oral antibiotic to prevent the disease.
• Haemophilus influenzae (haemophilus). Haemophilus influenzae type b (Hib) bacterium was once the leading cause of bacterial meningitis in children. But new Hib vaccines have greatly reduced the number of cases of this type of meningitis.
• Listeria monocytogenes (listeria). These bacteria can be found in unpasteurized cheeses, hot dogs and lunchmeats. Pregnant women, newborns, older adults and people with weakened immune systems are most susceptible. Listeria can cross the placental barrier, and infections in late pregnancy may be fatal to the baby.

Viral meningitis

Viral meningitis is usually mild and often clears on its own. Most cases in the United States are caused by a group of viruses known as enteroviruses, which are most common in late summer and early fall. Viruses such as herpes simplex virus, HIV, mumps virus, West Nile virus and others also can cause viral meningitis.

Chronic meningitis

Slow-growing organisms (such as fungi and Mycobacterium tuberculosis) that invade the membranes and fluid surrounding your brain cause chronic meningitis. Chronic meningitis develops over two weeks or more. The signs and symptoms of chronic meningitis — headache, fever, vomiting and mental cloudiness — are similar to those of acute meningitis.

Fungal meningitis

Fungal meningitis is relatively uncommon in the United States. It may mimic acute bacterial meningitis. It's often contracted by breathing in fungal spores that may be found in soil, decaying wood and bird droppings. Fungal meningitis isn't contagious from person to person. Cryptococcal meningitis is a common fungal form of the disease that affects people with immune deficiencies, such as AIDS. It's life-threatening if not treated with an antifungal medication. Even with treatment, fungal meningitis may recur.

Parasitic meningitis

Parasites can cause a rare type of meningitis called eosinophilic meningitis. Parasitic meningitis can also be caused by a tapeworm infection in the brain (cysticercosis) or cerebral malaria. Amoebic meningitis is a rare type that is sometimes contracted through swimming in fresh water and can quickly become life-threatening. The main parasites that cause meningitis typically infect animals. People are usually infected by eating foods contaminated with these parasites. Parasitic meningitis isn't spread between people.

Other meningitis causes

Meningitis can also result from noninfectious causes, such as chemical reactions, drug allergies, some types of cancer and inflammatory diseases such as sarcoidosis.