Most cases of meningitis can be classified as one of the two primary types – viral and bacterial – or a third type known as chronic meningitis:

• Viral (aseptic) meningitis. The most common form of meningitis, it affects all age groups but is especially prevalent in children. The U.S. Centers for Disease Control and Prevention (CDC) estimates that 25,000 to 50,000 Americans are hospitalized with viral meningitis every year. Most patients with viral meningitis, though, have symptoms that go away without any special treatment, and virtually all patients eventually make a complete recovery. However, symptoms of viral meningitis often 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, such as those that cause poliomyelitis) or other viral conditions (e.g., herpes simplex virus, human immunodeficiency virus). At least 20 viruses cause viral meningitis. At one time, mumps was a major cause of viral meningitis. However, immunization with the measles, mumps and rubella (MMR) vaccine has reduced such infections significantly.
  
  
• Bacterial meningitis. Usually the most severe type of meningitis, it is less common than viral meningitis and more likely to be life-threatening. Infants, young children and the elderly are most at risk. College students and others who spend time in close quarters with others (e.g., childcare facilities, dormitories, military units) also are at higher risk.
  
  Many types of bacteria can cause bacterial meningitis if they travel through the bloodstream and enter the brain. The most common causes of bacterial meningitis in newborns include Group B Streptococcus (GBS), Escherichia coli (E. coli) and Listeria monocytogenes. Streptococcus pneumoniae (pneumococcus) and Neisseria meningitidis (meningococcus) bacteria are more common in children older than 2 months of age. At one time, Haemophilus influenzae type B (Hib) was the leading cause of meningitis in children. However, use of the Hib vaccine has sharply curtailed this type of infection.
  
  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, such as from a nearby ear infection or sinus infection (sinusitis) or from a severe head injury such as a skull fracture. Left untreated, bacterial meningitis may cause brain damage, deafness, visual impairment, seizures, cognitive impairment (including learning disabilities) and can even be fatal. The adrenal glands, heart and kidneys also may be affected.
  
  
• Chronic meningitis. Inflammation of the meninges that is ongoing or recurrent. Children with chronic meningitis experience spontaneous episodes of severe headache and fever, often interrupted by extended periods without painful symptoms. Chronic meningitis has no known underlying cause, though it occurs relatively frequently in people with compromised immune systems (e.g., individuals with HIV/AIDS).