If I have a fever, vomiting, or have diarrhea, I cancel. If it's a morning gig, I try and have a voice mail or email waiting for the AD when they arrive in the morning to give them a heads up.

If its an afternoon gig, I'll make the call by 10:30am or so...

Usually, they're great about rescheduling...

If I'm just feeling generally crappy or I don't have a voice, I'll contact the AD and be straight with them. Sometimes they say background music is fine, other times they say stay home.