Basic Facts on Meningitis
Find out more about different causes of meningitis

Meningitis is commonly defined as an inflammation (usually caused by bacteria or viruses) of the meninges, the membranes that cover the brain and spinal cord. Other rare causes of meningitis include fungi, protozoa, and other parasites. Sometimes certain medications, cancers, or other diseases can inflame the meninges, although such noninfectious cases of meningitis are much rarer.
Although the bacteria and viruses that cause meningitis are fairly common and are more often associated with other everyday illnesses, sometimes they spread to the meninges from an infection in another part of the body. The infection can start anywhere, including in the gastrointestinal tract, skin, or urinary system, but the most common source is the respiratory tract. Microorganisms can enter the bloodstream, travel through the body, and enter the central nervous system.
In some cases of bacterial meningitis, the bacteria spread directly to the meninges from a severe nearby infection, such as a middle ear infection or nasal sinus infection. Bacteria may also enter the central nervous system after severe head trauma or head surgery.
Although bacterial meningitis is less common than viral meningitis, it is usually much more serious and can be life-threatening if not treated promptly. Many different types of bacteria can cause meningitis: Escherichia coli, Group B Streptococcus, and Listeria monocytogenes are the most common causes of meningitis in newborns. Streptococcus pneumoniae (pneumococcus) and Neisseria meningitidis (meningococcus) are more frequent in children older than 2 months of age. Haemophilus influenzae type b (Hib) was the leading cause of meningitis in children in the United States before the 1990s. The frequency of meningitis caused by Hib has dramatically decreased with the widespread use of the Hib vaccine as a routine childhood immunization
Viral meningitis, which often remains undiagnosed because its symptoms are similar to those of the common flu, is relatively common and far less serious than bacterial meningitis. Because people are more often exposed to common viral agents during those seasons, the frequency of viral meningitis increases slightly in the summer and fall months. Most cases of viral meningitis are associated with enteroviruses - viruses that typically cause stomach "flu." Other types of viruses, such as the herpes virus, can also cause meningitis. The mumps virus, once a common cause of viral meningitis, is now rare due to the routine use of the mumps, measles, and rubella (MMR) vaccine.
Although bacterial meningitis occurs in people of all ages, is more common in the young, especially infants and young children, and the elderly, defined as people above age 60. And because of time spent in close contact with many of their peers, teenagers and college students are also slightly more at risk for the disease. Viral meningitis occurs in people of all ages, although it is more common in children.