I think if Adam, Edge, or Larry became sick immediately before or during a show, or there was some reason they couldn't cancel the show (as with the Sydney show, for example, because they needed to work out the blocking for the next show's recording), Stuart, Dallas, or Sam could fill in. I guess if Bono's shoulders mysteriously fell off and he could no longer hold his guitar while he talked, Phil would step in to hold it for him
Otherwise, the band would try to reschedule, since they know the fans are paying to see them and not their techs.
The techs play together before sound check so Joe can adjust the sound system to the quirks of whatever venue they're in. If the band can't get to the venue in time for a sound check, the techs will do a full sound check for them (as was the case at the Buffalo show I saw a few years ago--because of weather, the band didn't arrive until the opening act was already on
). So all of them know the songs, and they could play them, but it's one of those things where I bet they learn them and hope they never need them.
I'm sure all of the band members have played through being sick before. They're not one of those acts who's constantly cancelling and rescheduling shows. They reserve postponing for extreme illness or emergencies; otherwise, the show must go on.