An inflammation of the meninges, most often caused by infection.

Meningitis is a potentially fatal inflammation of the meninges, membranes which encase the brain and spinal cord. Meningitis is most commonly caused by an infection of bacteria, viruses, or fungi, although there can be other causes, including bleeding into the meninges, cancer, or diseases of the immune system. The inflammation causes swelling of the brain, whose fragile tissues are pressed against the skull. Brain cells in these areas can become damaged and eventually die.

The most classic symptoms of meningitis include fever, headache, vomiting, photophobia (extreme sensitivity to light), irritability, lethargy, and a stiff neck. The disease progresses with seizures, confusion, and eventually coma. Infants, however, may not show these signs. A baby's immune system is not yet developed enough to mount a fever in response to infection, so fever may be absent. Some infants with meningitis may...