Story highlights Mumps causes swelling of the salivary glands and cheeks

It's spreading so fast at Harvard, it could affect commencement on May 26

The university first announced mumps cases in February

(CNN) Please stop infecting one another.

That is Harvard University's message to its students after a mumps outbreak left 40 people sick over the past two months.

Paul J. Barreira, director of the Massachusetts university's health services, told the student newspaper that the rise in cases is worrying.

"I'm actually more concerned now than I was during any time of the outbreak," Barreira told The Harvard Crimson. "I'm desperate to get students to take seriously that they shouldn't be infecting one another."