(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.
The university first announced mumps cases in February, and infections have steadily increased despite efforts to isolate patients.
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."
Mumps is spreading so fast, it could affect the university's May 26 commencement, he said.
"If there's a spike this week, that means those students expose others, so now we're looking at a potential serious interruption to commencement for students," Barreira told the student newspaper. "Students will get infected and then go into isolation."
Last month, the public health department in Cambridge, where Harvard is located, said all students affected had been immunized against mumps before they contracted it.
5 myths surrounding vaccines -- and the reality
Mumps is a viral infection that causes swelling in the salivary glands and cheeks, ear ache, jaw pain and general symptoms such as fever, muscle aches and headache.
The virus can be transmitted through respiratory droplets and contact with saliva or mucus.
Most mumps patients completely recover.
Recent U.S. outbreaks of mumps have mostly affected high schools and college campuses.
About three years ago, smaller outbreaks were reported in California, Virginia and Maryland.
Between 2009 and 2010, two large outbreaks occurred in New York, affecting about 3,000 people -- most of them high school students.