Encephalitis is an inflammation of the brain. It usually results
from an infection, most often by a virus, but sometimes by
bacteria, a fungus, or parasites. In rare cases, it is caused by
brain injury, a drug or vaccine reaction, or poison. A virus, or
other germ in the blood stream, can be carried to the brain.
Germs in the brain attract white blood cells, the body's main
line of defense against invaders, and this sets up an
inflammatory reaction. The brain tissue then swells (called
cerebral edema); bleeding may occur within the brain (called
intracerebral hemorrhage), and brain damage may occur. About 1
in 200,000 people develop encephalitis each year in the United
States. While anyone can succumb, children, the elderly, and
those with weakened immune systems are more vulnerable. Facts
About Encephalitis