Two medical advances hold promise for the 3.5 million Americans who each year come down with chicken pox.

Acyclovir, an antiviral drug used to treat some kinds of herpes and viral encephalitis, prevents the virus that causes chicken pox from multiplying, explained Dr. Keith Krasinksi, a pediatrician at New York University Medical Center.In addition, a vaccine for chicken pox has shown promise in animals, adults and immune-suppressed children. If proven safe and effective, the vaccine could be used in all children and adults.

"We are not now recommending acyclovir for the average child with chicken pox," Krasinski said. "However, this may change as we gain more experience using the drug with children with normal immune systems. We do use it for patients with impaired immune systems, in whom chicken pox may produce serious disease."

A study in normal children with chicken pox showed those taking oral acyclovir had fewer lesions and lower, shorter-lasting fevers.

Although generally an annoying week of itching and fever, chicken pox can be life-threatening in children or adults with impaired immune systems.

In rare instances, complications occur in otherwise healthy people, including severe skin irritation, pneumonia or encephalitis. In the United States, about 4,000 people are hospitalized and about 150 people die from chicken pox each year.

Usually a childhood disease, 95 percent of Americans contract it before the age of 18. But chicken pox also can affect adults.

Until now, there has been no definitive treatment for chicken pox. Children and adults are treated with medications to reduce fever, and occasionally with topical medication to control itching.

Krasinski cautioned that aspirin should not be used in children with chicken pox. The combination of the virus and aspirin use has been associated with Reye's syndrome, which may lead to coma and death.

Chicken pox is a viral infection caused by the varicella zoster virus, a member of the herpes family.

One bout confers lifelong immunity against chicken pox. However, the same virus causes shingles, an often painful viral inflammation of the skin.

When chicken pox subsides, the virus lies dormant in sensory nerves. It may become reactivated in response to changes in the immune system, resulting in shingles.

Symptoms of chicken pox, which may be more severe in adults, include fever and a rash of tiny lesions called pocks, which fill with fluid and then become encrusted.