That's more than the combined total of cases in the five years prior.
Several cases of those mumps have been reported in Union County in Northeast Oregon.
The easily-spread virus causes puffy cheeks and a swollen jaw, as well as fever, muscle aches and loss of appetite. Union County has four confirmed and five likely cases of the disease.
"Our total population in the county is 25,000 people," said Andi Walsh with the Center for Human Development agency in La Grande.
"Having three cases is what we consider an actual outbreak. So this is considered serious for us."
Mumps is a vaccine-preventable disease.
The Oregon Health Authority said public health officials are working to identify the sources of this outbreak and to identify exposed persons.
Mumps cases are no longer very common in the U.S., according to the Centers for Disease Control and Prevention.
The CDC says cases in the U.S. have declined 99 percent because of vaccines.
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