Some 56 cases of mumps have been reported in Ohio in an outbreak that started at Ohio State University in Columbus but has spread to people without ties to the school — a “disturbing development,” say public health officials.

Forty of those infected are Ohio State students, staff members or people with close ties to the school’s campus, but the most recent cases have no connection to the university, said Jose Rodriguez, a spokesman for the Columbus health department.

Rodriguez said the majority of those infected had received at least one round of the vaccinations for measles, mumps and rubella and it will be difficult to pinpoint where the outbreak began.

“It is an easy disease to give and get, much like the flu or cold, and it spreads in highly populated areas,” Rodriguez said.

Franklin County, which includes Columbus, typically sees one reported mumps case per year, Rodriguez said.

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, according to the U.S. Centers for Disease Control and Prevention.