The Enigma Strain (Harvey Bennett #1)

ASIN moved from less recent edition here

When a bomb goes off at Yellowstone National Park, the reclusive park ranger Harvey "Ben" Bennett is startled to realize that the blast wasn't the only threat: the bomb released some sort of virus into the air, and it's beginning to infect and kill more and more people.

It's a race against time as Ben teams up with a woman from the CDC to figure who's behind the attacks, and how to stop the next one.