An outbreak of mumps is spreading like wildfire across at least seven midwestern states.

Missouri has four cases. There are 35 confirmed cases in Illinois; up from 24 just two days ago. And Iowa has more than 600 cases.

The latest confirmations are in Peoria County and at Southern Illinois University in Carbondale where three students have been affected. All have been sent home for at least one week.

The outbreak has prompted action on SIU''s sister campus in Edwardsville. There have been no cases of mumps there but experts say viruses can spread quickly on college campuses because people are living in close quarters.

Officials say 98-percent of Edwardsville''s 13,000 students have been vaccinated. The vaccine is commonly known as M.M.R. for measles, mumps and rubella.

The latest outbreak, the biggest in 20 years, mainly affects people ages 18 to 25.

Mumps is spread by sneezing and coughing.

Symptoms include fever, headache and swollen saliva glands.

There can be serious side affects including a loss of hearing, meningitis and in men, swollen testicles. Physicians say getting vaccinated is still your best bet but unless you''re vaccinated the right way, you could still be vulnerable. Dr. Keith Woeltje of Barnes-Jewish Hospital and an associate professor at the Washington University School of Medicine, says, "The people at highest risk are the people who''ve never been vaccinated. If they''re exposed, they have a very high chance of developing the mumps. People who''ve only gotten one shot may have some protection, but again, their risk is much higher than people who''ve gotten both shots of the vaccine series."

He adds that if you''ve been exposed to the mumps, monitor your symptoms, contact your doctor and try to avoid contact with other people.