The central nervous system consists of the brain and spinal cord. These organs are remarkably resistant to infection, but when they become infected, the consequences are often very serious. Infections may be caused by bacteria, viruses, fungi, or, occasionally, protozoa or parasites. Another group of brain disorders that resemble infections, called spongiform encephalopathies, are caused by prions, which are abnormal protein molecules (see Prion Diseases: Overview of Prion Diseases).
Infections usually cause inflammation. For example, infection can cause encephalitis, which is inflammation of the brain. Infections that involve the fluid-filled space within the layers of tissue (meninges) that cover the brain and spinal cord are called meningitis (see see Meningitis). Often, bacterial meningitis spreads to the brain, causing encephalitis. Similarly, viral infections that cause encephalitis often also cause meningitis. Thus, usually when bacterial meningitis or viral encephalitis develops, the resulting disorder is technically meningoencephalitis. However, infection that affects mainly the subarachnoid space and meninges is usually called meningitis, and infection that affects mainly the brain is usually called encephalitis.
In meningitis and encephalitis, inflammation occurs throughout the brain and, in meningitis, throughout the spinal cord. But sometimes infection is confined to one area (localized) as a collection of pus, called an empyema or an abscess depending on where it is located. An abscess, which resembles a boil, can form anywhere in the body, including the brain. Fungi (such as aspergilli), protozoa (such as Toxoplasma gondii), and parasites (such as Taenia solium may cause a localized brain infection similar to an abscess.
Bacteria and other infectious organisms can reach the meninges and other areas of the brain in several ways:
Last full review/revision May 2008 by Michael Jacewicz, MD