Illinois Department of Public Health Director Eric Whitaker says Illinois -- starting in 1988 -- has required college students to have had vaccinations for mumps or prove they're immune because they've already had the illness.
That adds to the state's "herd immunity" -- which means vaccinated people form a barrier protecting others from the virus.
The number of mumps cases in the state continues to climb. It's now 123 cases, including 66 confirmed and 57 suspected ones.
Iowa has more than 900 cases.
Mumps can spread through coughing and sneezing. Symptoms include fever, headache and swollen salivary glands.