Mumps cases are spilling out of Iowa, popping up in at least seven other Midwestern states including Wisconsin and perhaps seven more.
The outbreak is leading to promises of extra vaccine from the US stockpile.
There are no deaths and few complications being reported from this virus, which health officials believe got a boost from air travel.
But the nation's federal health agency says it's the largest outbreak in almost two decades with more than a thousand cases. It's expected to keep growing.
More than 800 of the cases are in Iowa, and a dozen are in Wisconsin.
The Centers for Disease Control and Prevention has pledged to provide 25-thousand doses of measles-mumps-rubella vaccine to Iowa from the agency's stockpile.
And vaccine maker Merck is giving another 25-thousand doses to the C-D-C for distribution to other states.