What Is Meningitis?

Meningitis is an inflammation of the meninges, the lining that protects the brain and spinal cord.

It is almost always caused by an infection, usually by a bacteria (bacterial meningitis) or a virus (viral meningitis). In rare cases it can be triggered by a fungus or parasite.

Meningitis occurs most commonly in young children under 5, those aged 17-25 (who often live in close quarters like dormitories and barracks), and people over age 55. People with compromised immune systems, such as people with HIV or AIDS, are also at increased risk.

What Are The Meninges?

The meninges are composed of three layers of membranes enclosing the brain and spinal cord.

• Pia mater is the innermost layer. It is akin to a tissue paper that closely adheres to the brain and spinal cord, dipping into the various folds and crevices.

• Arachnoid mater is the middle layer. It is a filmy membrane that is joined to the pia mater by fine threads resembling a cobweb.

• The dura mater, a parchment-like membrane, lies on the outermost part of the meninges and adheres to the skull and spinal canal.

• The cerebral spinal fluid (CSF) is the fluid that circulates in the spaces in and around the brain and spinal cord.

What Happens When The Meninges Swell?

The most frequent cause of meningitis is the entry of a microorganism-such as a bacterium or a virus-from an infection elsewhere in the body. The microorganisms travel through the blood and into the meninges and cerebral spinal fluid.

In the bloodstream, infection-causing microorganisms are fought off by white blood cells, an important part of the immune system. However, there are no white blood cells in the cerebral spinal fluid to fight infectious agents.

Once infectious organisms have entered the cerebrospinal fluid, the body's defenses cannot control their rapid growth and the disease races through the delicate surfaces and fluids of the central nervous system.

As the immune system gears up to fight off the microorganisms, it sends out chemical signals that produce inflammation and interfere with the normal functioning of the central nervous system. That, in turn, causes swelling and increased pressure inside the skull, and disrupts the brain's normal functioning.