polyphemus moth caterpillar (sick)

This poor caterpillar was lying by the side of a foot path north or Durango, CO. I'm pretty sure it's a polyphemus moth caterpillar like the one KarenL posted a couple of weeks ago (http://www.projectnoah.org/spottings/7375653). But unlike the one she found, which looked very healthy, this one appeared to be dead until I looked closely and saw that it could move a little. I don't know what was wrong with it.