What is Illness That Keeps Letterman Out Sick?

March 3 -- The eye infection that has kept Late Show host David Letterman off the air is linked to shingles, an ailment caused by the same virus that causes chickenpox in children.

"It's an infection caused by one of the members of the herpes family of viruses, the same virus that causes chickenpox when we're young," ABCNEWS' Dr. Tim Johnson said.

"So if we've had the chickenpox, the virus stays in our body, hides out in nerve cells along [the] spinal column, and for reasons that are sometimes clear and more often not clear, the virus re-awakens in later life, and activates along the nerve cells."

Because of Letterman's illness, various hosts filled in last week, including Bruce Willis, who had been scheduled as a guest Wednesday night. Except for when he missed some air time time after a heart bypass operation, this is the first time Letterman, 53, has called in sick in 20 years, a representative for the CBS show said.

Itching and Blisters

The symptoms of shingles include initial itching and pain, along the course of a spinal nerve, almost always on just one side of the body. That is followed by rash, and actual fluid-filled blisters. The blisters are sometimes followed by post-shingles pain that continues for many months after the blisters go away.

Typically, the pain goes away in 4 to 6 weeks, but especially in older people there can be post-shingles pain, sometimes very severe for many months, Johnson said.

The only way to contract shingles is if you had the chicken pox as a child, and still have the virus in your system. Any former chicken pox sufferers can get it, but the elderly, or people with compromised immune systems, such as cancer or AIDS patients, are more susceptible. Unlike the chicken pox, which can spread to people who have never had it, shingles is not contagious.

Treatment Needed Quickly

It's relatively rare for shingles to appear on the upper part of the face, as is the case with Letterman, Johnson said. But when patients do experience shingles on their upper face, it can affect the eyes and in the worst-case scenario, it can lead to blindness.