Meningitis occurs when the meninges, the membranes covering the brain and spinal cord become inflamed. Most cases result from bacterial, viral or fungal infection. Rarely, meningitis may also occur in reaction to physical trauma, or some medications or diseases (e.g., cancer). Anyone can develop meningitis, although young children are particularly susceptible to the disease. Children under the age of 2 years are especially vulnerable to meningitis because their immune systems are not yet fully developed.

Other groups that are especially vulnerable include older adults and individuals with a long-standing health condition (e.g., HIV/AIDS). Though meningitis may be potentially life-threatening, children who have this disease typically make a full recovery when it is diagnosed and treated promptly.

The central nervous system is covered by three membranous layers of connective tissue called meninges. The meninges protect neural tissues of the brain and spinal cord, as well as enable the vertebral column and spinal cord to flex and twist.

The inflexible outermost layer of the meninges is the dura mater. It fuses with the lining of the skull. The middle layer of the meninges is the arachnoid mater. This membrane is separated from the delicate, innermost membrane (pia mater) by a space that contains cerebrospinal fluid (CSF). This fluid carries nutrients, helps absorb impact and helps defend the brain from harmful microorganisms.

In many cases, meningitis stems from bacteria, viruses or fungi that are associated with everyday illnesses. In other cases, infections that result in meningitis begin in places such as the respiratory tract, skin, gastrointestinal tract and urinary system. Bacteria, viruses or fungi reach the meninges through the bloodstream, from nearby infections (e.g., sinusitis) or by direct contact (e.g., penetrating injury, surgical procedure).

While in the bloodstream, these microorganisms resist attack by white blood cells, causing receptors in the brain to allow penetration into the CSF. Once in the CSF, infection tends to progress rapidly because of the lack of disease-fighting substances (antibodies). The resulting inflammation causes exudate (oozing fluid) to damage cranial nerves and pathways that carry signals to other parts of the body. It also causes swelling of the meninges, which disrupts the normal flow of blood and oxygen to the brain.

Parents of children who exhibit symptoms associated with meningitis are urged to seek prompt medical care. The sooner meningitis is diagnosed and treated; the less severe the health consequences are likely to be.

However, the complications associated with meningitis can be severe. Even with effective antimicrobial therapy, significant neurological complications have been reported in some patients following an episode of bacterial meningitis. An increased intracranial pressure (ICP) is a potentially fatal complication. The main sign of increased ICP is an altered state of consciousness, which may vary from lethargy to a state of prolonged unconsciousness, which can rapidly progress to death. Other severe complications include cerebral palsy, mental retardation, blindness, seizures, adrenal gland failure, hearing impairment and/or loss of speech, learning disabilities and behavioral problems.