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.
I agree, Bill. If I'm sick, the last thing the AD, or myself, wants is for me to be in there with the high probability of the residents contracting whatever bug I'm ill with. Like yourself, I give them the heads up at the earliest possible opportunity, (most times you know a day or two in advance), and any AD worth their salt will be very happy with that and will reschedule the gig, no problem.