Mumps, once common, was on track to be eliminated in the U.S. by the year 2010, thanks to widespread use of two doses of the measles-mumps-rubella vaccine in early childhood. Then the largest U.S. mumps epidemic in two decades occurred in 2006.

In a study reported recently in the New England Journal of Medicine, researchers from the Centers for Disease Control and Prevention and state health departments examined the 6,584 cases in the Midwestern outbreak. They found that the incidence was four times higher among people 18 to 24 years old than in all other age groups combined. In addition, 84 percent of the patients in that age group had received the mumps vaccines.

"Close-contact living conditions, like on college campuses, helped spread the disease," says Jane Seward, the CDC's deputy director of the division of viral diseases and an author of the study. But it also appears that there was a decrease in immunity.

Dr. Seward says the 2006 outbreak would have been much larger if so many people had not received vaccinations.

"If we see sustained transmission again, we would offer a third dose. Two doses of MMR vaccine are very effective, but not 100 percent effective."