The Heartland virus was considered rare. Scientists now say they've found signs of it circulating in animals across the Midwest, New England and the South. They think human cases have been missed.

Last year, virologists traced the mysterious illness of two Missouri farmers to a virus never seen before. Now, scientists have found the so-called Heartland virus in ticks. The discovery means the U.S. has another tick-borne illness on its hands — and "another reason to avoid getting bit."

So far, two Missouri farmers are the only known cases of the tick-borne virus in the world. But experts are sure they'll find more. The men recovered but suffered serious illness that required hospital care and weeks of convalescence.