Heather Swanson and Ryan Prioreschi stand in knee-high golden grass on a slope outside Boulder, Colorado, where the Rocky Mountains start slumping into the plains, at the epicenter of a now-international animal epidemic. The two ecologists, who monitor wildlife for the city, have their binoculars out, and they’re staring right at the problem.  

A fawn runs circles around the rest of the herd, with the boing of a muscular slinky toy. 

“He’s wired,” says Swanson, laughing. “He’s doing laps.”

A few other mule deer rear up on their hind legs and kick each other. Still others just hang out in the shade. It’s a beautiful spring morning and the animals look sleek and healthy. But all is not what it seems. This herd is harboring an infection — chronic wasting disease, or CWD.

Scientists have called this neurodegenerative disease, which attacks deer, elk and moose, a “nightmare” and a “state of emergency.” Lately, the media’s been calling it “zombie deer disease.”