Infections of the brain can be caused by viruses, bacteria, fungi, or, occasionally, protozoa or parasites. Another group of brain disorders, called spongiform encephalopathies, are caused by abnormal substances called prions.
Infections of the brain often also involve other parts of the central nervous system, including the spinal cord. The brain and spinal cord are usually protected from infection, but when they become infected, the consequences are often very serious.
Infections can cause inflammation of the brain (encephalitis). Viruses are the most common causes of encephalitis. Infections can also cause inflammation of the layers of tissue (meninges) that cover the brain and spinal cord—called meningitis. Often, bacterial meningitis spreads to the brain itself, causing encephalitis. Similarly, viral infections that cause encephalitis often also cause meningitis. Technically, when both the brain and the meninges are infected, the disorder is called meningoencephalitis. However, infection that affects mainly the meninges is usually called meningitis, and infection that affects mainly the brain is usually called encephalitis.
Usually in encephalitis and meningitis, infection is not confined to one area. It may occur throughout the brain or within meninges along the entire length of the spinal cord and over the entire brain.
But in some disorders, infection is confined to one area (localized) as a pocket of pus, called an empyema or an abscess, depending on where it is located:
Fungi (such as aspergilli), protozoa (such as Toxoplasma gondii), and parasites (such as Taenia solium) may cause cysts to form in the brain. These localized brain infections consist of a cluster of organisms enclosed in a protective wall.
Sometimes a brain infection, a vaccine, cancer, or another disorder triggers a misguided immune reaction, causing the immune system to attack normal cells in the brain (an autoimmune reaction). As a result, the brain becomes inflamed. This disorder is called postinfectious encephalitis.
Bacteria and other infectious organisms can reach the brain and meninges in several ways: