The complication is eczema vaccinatum, a rash that can spread all over the body and, in the worst case, be fatal. Having atopic dermatitis or eczema puts a small percentage of people who get smallpox vaccine at risk of the condition.

But when epidemiologists at the Marshfield Clinic Research Foundation in Wisconsin did a telephone survey of area residents who had been diagnosed and treated for the conditions from 1979 through 2001, they found that 30 percent to 40 percent of them didn't remember, says a new report. The study, which will appear in the July 1 issue of the Annals of Internal Medicine, was released online May 5.

"Our study used people who had two clinic visits with diagnoses separated by at least 60 days," says Dr. Edward A. Belongia, an epidemiologist at the clinic. "We found that among adults who had had atopic dermatitis, only 59 percent were able to report it."

The results were a little better for children, with 30 percent of parents recalling their diagnoses. Children are in danger of eczema vaccinatum even if they aren't vaccinated, since the vaccine virus can be passed from someone who has had the vaccine to members of the household.

The questions asked in the telephone survey were the same recommended by the U.S. Centers for Disease Control and Prevention (CDC) in the government's program for nationwide smallpox vaccination, Belongia says.

The risk is not overwhelming -- the incidence of the complication in the 1960s, when vaccination was routine, was 40 per million -- but "if you run a program to vaccinate hundreds of thousands of people, it is important to realize that people who have had these skin conditions that defer them from having the vaccine cannot accurately recall having those conditions," he says.

The problem could affect a lot of Americans, because it's estimated that as many as 20 percent of people in this country may have some form of dermatitis, says Dr. Samuel Katz, professor emeritus of pediatrics at Duke University, who serves as liaison between the Infectious Diseases Society of America and the CDC on smallpox vaccination.

On the other hand, Americans seem to be forgetting the smallpox program rapidly, Katz says. The goal was to vaccinate 500,000 people; it has reached about 35,000, he says. And with Iraq conquered, fears of that weapon of mass destruction have been disarmed, Katz says.

"We may not get many more," he says. "The climate has changed enormously. It has in our hospital. There is a great diminution of anxiety. We were getting a great number of telephone calls, and now we aren't."

Telephone calls are still coming in, but about a totally different reason to worry, Katz says. "Now the whole focus in on SARS," the new and deadly respiratory infection, Katz says. "People have forgotten about smallpox."

More information

All you ever want to know about smallpox is available from the Centers for Disease Control and Prevention. For more on eczema, click on the American Academy of Dermatology.