In 1964, at age 20, skier Jimmie Huega won the Olympic bronze medal in the slalom. In 1967, his vision blurred and then his legs became slightly numb. Jimmie ignored these intermittent symptoms, and after a while they disappeared.

Three years later, the symptoms returned, and this time Jimmie sought medical help. On the basis of his symptoms, which affected more than one body part and occurred sporadically, physicians diagnosed multiple sclerosis (MS). Today, diagnosis also includes a magnetic resonance imaging scan, which can detect brain lesions.

Jimmie can still ski and cycle and do situps and pushups, but he never knows when the on-again off-again disorder will strike. For many of the 300,000 people in the United States with MS, the progressive deterioration causes permanent paralysis. Fortunately, taking the immune system biochemical beta interferon as a drug can prevent flare-ups and mitigate symptoms.

In MS, the myelin coating in various sites through the brain and spinal cord forms hard scars called scleroses. The neurons that they surround can no longer transmit messages. Muscles that no longer receive input from motor neurons stop contracting. Without stimulation, muscles atrophy. Symptoms depend upon which neurons are affected. Short-circuiting in one part of the brain may affect fine coordination in one hand; if another brain part is affected, vision may be altered.

What might be responsible for the destruction of myelin in MS? A virus may cause the body's immune system to attack the cells producing myelin. This would happen if viruses lay latent in nerve cells, then emerged years later bearing proteins also found on nerve cells. The immune system, interpreting the proteins as foreign, would attack the viruses as well as the neurons.

A virus is suspected for a few reasons: viral infections are known to strip neurons of their myelin sheaths; viral infections can cause repeated bouts of symptoms; and most compelling, MS is far more common in some geographical regions (the temperate zones of Europe, South America, and North America) than others, suggesting a pattern of infection. A possible culprit is the virus that causes measles.