Don't
If you wanna risk killing a good gig. Reschedule, find a replacement, do something. You can't be GREAT sick. Unless your a superstar and running your show through a pro tools rig, your audience is gonna notice your voice is shot, they don't care if you're sick, they're just gonna say, man, that singer blows chunks. Save yourself, save your band, save your paying gig. Don't sing sick if at all possible. Seriously. Not cool.
AND THAT DUDE ON THAT VIDEO PROVES MY POINT.
Great find (gross and cool all at the same time).