A mumps outbreak that started in Iowa and has been most evident at colleges is spreading outside the state to colleges throughout the Midwest and as far away as Pennsylvania.

While most people who have mumps do not experience serious damage, mumps can result in significant harm, especially if untreated. And the close quarters in which college students live and interact can make campuses vulnerable to fast spreading outbreaks, according to a joint statement issued last week by the Centers for Disease Control and Prevention and the American College Health Association.

In a briefing, Julie Gerberding, director of the CDC, said that the outbreak was the worst in the United States in 20 years. While a mumps vaccine has been widely available since 1967, she said that not all people, and not all college students, have had it in the two doses that make it most effective. She also noted that the vaccine is not effective with all people.

The CDC and the college health group urge colleges facing an outbreak to isolate students who have been diagnosed, and to encourage vaccinations. The CDC is sending extra vaccines to affected campuses, especially in Iowa. In addition, the agency is investigating the way mumps spread, to look for clues as to why this outbreak took place and how it might be blocked from spreading further.

The University of Iowa has more than 50 students diagnosed with mumps and is offering vaccinations. So is Iowa State University, which to date has had only three cases. One employee in the Eastern Iowa Community College District contracted mumps, but is recovering, with no indication that others in the district have symptoms.

Among those investigating possible outbreaks outside Iowa are Washburn University, in Topeka, where seven possible cases are under investigation; Dakota State University, in South Dakota, where one case has been reported; Indiana University at Bloomington, and several Milwaukee colleges.

Franklin & Marshall College, in Pennsylvania, has confirmed two cases and expects to confirm four more.