Meningitis can be classified into three types:

• Viral (aseptic) meningitis. The most common form of meningitis, it tends to be relatively mild. Each year, between 25,000 and 50,000 cases of viral meningitis require hospitalization, according to the U.S. Centers for Disease Control and Prevention (CDC). Most patients with viral meningitis have symptoms that go away without any special treatment. However, symptoms of viral meningitis may closely resemble those associated with bacterial meningitis, a much more serious form of the disease that requires immediate treatment. This similarity may complicate the process of diagnosing the source and severity of the infection. Viral meningitis may be caused by enteroviruses (viruses that infect the intestinal tract) or other viral conditions (e.g., herpes simplex virus, human immunodeficiency virus).
  
  
• Bacterial meningitis. Usually the most severe type of meningitis. Many different types of bacteria can cause bacterial meningitis if they travel through the bloodstream and enter the brain. The most common forms of bacterial meningitis include the bacteria that cause pneumonia (pneumococcus) and meningococcus bacteria (Neissaria meningitides).
  
  

  According to the CDC, about 6,000 people develop pneumococcal meningitis and 2,600 people develop meningococcal meningitis in the United States each year. Vaccines are available that can prevent some types of bacterial meningitis from spreading to other people. The disease can also occur if bacteria invade the meninges directly (e.g., ear or sinus infections, skull fractures). Left untreated, bacterial meningitis may cause brain damage, deafness, cognitive impairment and even death.
  
  

• Chronic meningitis. Inflammation of the meninges that is ongoing or recurrent. Patients with chronic meningitis may experience spontaneous episodes of severe headache and fever, often interrupted by extended periods without painful symptoms. Most cases of chronic meningitis have no known underlying cause, though it occurs relatively frequently in people with compromised immune systems (e.g., due to HIV/AIDS, cancer, chemotherapy use, long-term use of the corticosteroid prednisone). It also sometimes occurs in people with healthy immune systems who contract tuberculosis (TB), Lyme disease or other infections. Bacterial meningitis that has been only partially treated (but not eliminated from the body) may lead to chronic meningitis. Drugs such chemotherapy medications, organ-transplantation medications and nonsteroidal anti-inflammatory drugs have also been associated with chronic meningitis.

In addition, certain fungi (e.g., cryptococcus) may cause meningitis. This is rare, however, and usually occurs in patients whose immune systems are severely weakened from disease (e.g., HIV/AIDS, cancer).