Meningitis

Meningitis is an inflammation of the meninges, the membranes that cover the brain and spinal cord. The inflammation is usually caused by bacteria or viruses (viral meningitis is also called aseptic meningitis). Less common causes 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.

Many of the bacteria or viruses that can cause meningitis are fairly common and are more often associated with other everyday illnesses. Sometimes, however, they spread to the meninges from an infection in another part of the body. The infection can start anywhere, including in the skin, gastrointestinal tract, or urinary system, but the most common source is the respiratory tract. From there the 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 serious ear infection (otitis media) or nasal sinus infection (sinusitis). Bacteria may also enter the central nervous system after severe head trauma or head surgery.

Bacterial meningitis is less common than viral meningitis but is usually much more serious and can be life-threatening if not treated promptly. Many different types of bacteria can cause meningitis: Group B Streptococcus, Escherichia coli, 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. Before the 1990s, Haemophilus influenzae type b (Hib) was the leading cause of meningitis in children in the United States. But the widespread use of the Hib vaccine as a routine childhood immunization has dramatically decreased the frequency of meningitis caused by Hib.

Viral meningitis is relatively common and far less serious than bacterial meningitis. It often remains undiagnosed because its symptoms are similar to those of the common flu. The frequency of viral meningitis increases slightly in the summer and fall months because people are more often exposed to common viral agents during those seasons. Most cases of viral meningitis are associated with enteroviruses - viruses that typically cause stomach "flu." However, many other types of viruses, such as the herpes virus, can also cause meningitis. The mumps virus was once a common cause of viral meningitis, but it is now rare due to the routine use of the measles, mumps, and rubella (MMR) vaccine.

Bacterial meningitis occurs in people of all ages but is more common in the very young (infants and young children) and the elderly (people above age 60). Teenagers and college students are also slightly more at risk for the disease because of time spent in close contact with many of their peers. Viral meningitis occurs in people of all ages, although it is more common in children.

People with less competent immune systems, such as the very young or those whose immune systems have been compromised by disease, are more at risk for all types of meningitis. Some types are more common in child-care centers, boarding schools, dormitories, and military bases, mainly because infectious diseases tend to spread quickly among large groups of people. Not receiving routine vaccines for certain infectious agents that can cause meningitis - including those for mumps, Hib, and pneumococcus - also increases a person's risk for the disease.