I think the best thing is to do your best to find a substitute performer yourself, unless the nature of the gig makes it really impossible. That way, you're not letting down the booker or even creating that much extra work for them, and your substitute owes you a favour and may call you when they need to suggest someone.

What I've done a couple of times, when I was mostly over a cold and felt that I could play but not sing yet, was hire a singer to go with me. Since I couldn't expect the venue to pay more than we had agreed for a last-minute addition, and since half of the one-person fee wouldn't be a fair price for the singer, I ended up giving the whole fee to the singer. But I felt that doing the shows for free was better for my reputation than cancelling.

You don't have to be at death's door to be too sick to perform, especially if you're solo and especially if there's no PA. Even if it's "just a cold" - you might be able to get away singing on a sore throat if there's a PA, but do you really want to be interrupting the songs to have a coughing or sneezing fit? And if you have a runny nose, how are you supposed to play guitar - just leave the snot on your face till the end of each song? Nobody wants to see that.

Oh, and about Big Mick's "the show must go on"... I'd think twice before taking advice from the guy who played through a heart attack...

Cheers, Marion