More than 1,000 cases have been reported in eight states, and suspected cases are being investigated in seven other states, Gerberding said Wednesday at an afternoon press briefing

"We're not going to be surprised if there are more cases in more states, just given the nature of mumps, and the way this outbreak is progressing."

Rarely Has Lasting Effects

The majority of cases, 815 so far, have been in Iowa, where the outbreak appears to have begun late last year at a university not identified by the CDC. About 350 cases have been reported to the CDC from seven other states: Minnesota, Kansas, Illinois, Nebraska, Wisconsin, Missouri and Oklahoma, Gerberding said. She would not identify the states where suspected cases are under investigation.

Mumps is a virus. It is spread easily through coughing or sneezing, or by touching contaminated surfaces. In up to one-third of cases, there are no symptoms, but in others it can cause fever, headache and swelling of the salivary glands that usually lasts five to nine days. In some cases, it can cause encephalitis and other conditions, but it rarely has lasting effects.

Twenty people have been hospitalized in the outbreak, but no one has died, the CDC says.

The Iowa Department of Public Health reports that 64% of the state's cases have been in people who got two doses of a measles-mumps-rubella, or MMR, vaccine.

"We have absolutely no information to suggest there's any problem with the vaccine," she says. "The problem is with the lack of complete coverage with the vaccine."

'Very Good Vaccine'

Since the early 1990s, most children have received mumps vaccine in two doses as part of the MMR shot, but many young adults have had just one shot. Even among those who got both doses, 10% may remain susceptible.

"This is a very good vaccine, but it's not perfect," she says.

Still, college-age people, and especially healthcare workers, should make sure they have had two doses, the CDC advises. The CDC has offered 25,000 doses of MMR vaccine from its stockpile to Iowa, and vaccine maker Merck has donated 25,000 doses to CDC to use as needed.

People who have mumps are advised to remain isolated to reduce the spread of the virus.

Mumps has become an uncommon disease since 1967, when the vaccine was licensed, and some doctors might never have seen it.

"As clinicians become more familiar, more aware of what we're looking for, we would expect to see more cases just because there are more cases being diagnosed," Gerberding said.

The last major outbreak was in Kansas in 1986, with 269 cases.