“Mumps outbreaks are on the rise,” said Dr. Janell Routh, a pediatrician who is a medical officer on the mumps team at the Centers for Disease Control and Prevention. More than 6,000 cases of mumps were reported in the United States last year, the highest number in 10 years. Around 2010, total annual cases were down in the hundreds.

Most of the recent cases occurred in outbreaks, including a large one in Arkansas, rather than as a sporadic here-a-case, there-a-case disease. And most of the outbreaks were among people 18 to 22 years old, most of whom had had the requisite two doses of mumps vaccine in childhood. “We are seeing it in a young and highly vaccinated population,” Dr. Routh said.