On Tuesday, Reuters reported that the Columbus Health Department said that number could rise further this week. Students began returning from spring break on Monday and five cases have been reported since then.
Jose Rodriquez, a spokesman for the city's health agency, said those infected include 23 students and a staff member at the university in Columbus, Ohio, and four other people with connections to students or links to the community.
All but one had reportedly received at least one dose of the mumps vaccine. The university does not require students to be vaccinated.
"The possibility is always there for more cases," Rodriguez said. "With the long incubation period mumps has, it is a recipe for a prolonged outbreak."
According to the Centers for Disease Control and Prevention, mumps is a contagious disease that causes painful swelling of the salivary glands. The number of cases reported annually in the United States has dropped 98 percent since the mumps vaccine was introduced in the 1960s.
Mumps cases are uncommon in the United States, but not unheard of on college campuses. A multi-state outbreak in 2006 led to nearly 6,600 reported cases with more than 80 percent of the people saying they were attending college.
Smaller outbreaks have since been reported, including a situation last month at two New York college campuses in Manhattan and the Bronx.
Rodriguez, along with a spokeswoman for the university, said the source of the outbreak had not been confirmed.