Encephalitis: Viral - Introduction

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[For more information, see the Causes section in this report.]

How Viruses Can Infect the Central Nervous System

Encephalitis can develop shortly after an initial viral infection, or it can develop when a virus that was lying dormant in the body suddenly reactivates. Viruses are simple, but powerful infectious organisms.

• The virus infects a person (host) by penetrating a cell membrane and ejecting its genetic material (its DNA or RNA) into the cell.
• The viral DNA or RNA takes control of important cell processes, telling the cell to make more viruses.
• The cell ruptures, releasing new viral particles that infect other cells.

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There are two ways that viruses can infect brain cells:

• The virus silently invades the body. There are no initial symptoms. The virus is carried by the bloodstream to the nerve cells of the brain, where they gather and multiply. Viruses that enter the brain in this manner are often widely scattered throughout the brain. This is called diffuse encephalitis.
• A virus first infects other tissue and then invades brain cells. Viruses that are transmitted from other tissues usually cause focal infection, meaning they produce extensive damage in only a small area of the brain.

The Central Nervous System

The brain and spinal cord comprise the central nervous system. The adult human brain weighs about 3 pounds (1.4 kilograms). There are two major parts of the brain:

• The higher and larger forebrain (the cerebrum)
• The lower and smaller brain stem

The Cerebrum

The cerebrum is the uppermost and largest part of the brain. It is the most highly developed section of the brain. There cerebrum has several components:

The Cerebral Cortex. The cortex is the outermost layer of the cerebrum. It is made of gray and white matter:

• Gray matter is a thin sheet of nerve cells that cover the surface of the brain.
• White matter is a bundle of insulated nerve fibers that underlies the cortex and makes up the core of the cerebral hemispheres.

The Hemispheres. The two hemispheres control higher brain functions, such as memory, learning, decision making, and processing input from the senses. They are each divided into four lobes, which regulate different brain functions: