MADISON, Wis. -- Health officials are keeping a close eye on a mumps outbreak that continues to spread across the Midwest.

Doctors say the mumps are so rare most people don't even recognize it. The virus swells the salviatory glands, creating a chipmunk look.

It usually goes away after a week, but it's highly contagious.

"We're seeing that most of our cases are occurring in the 18 to 25 years of age, and about half of them are college students," said Iowa State epidemiologist Dr. Patricia Guinlisk.

A state like Iowa sees about five cases a year. But this year there have been at least 600 cases reported, WISC-TV reported.

Another 100 cases have been reported in six other Midwestern states, including four in Wisconsin.

And even if you've been vaccinated for the mumps, you may not be in the clear. Doctors fear the shot may wear off for some, which could be allowing the illness to spread.