...when you've got a massive respiratory bug that has you hacking/blowing out nasties continuously, plus you have two unrelated uncomfortable conditions, and you're in the field in the rain.
Luckily, I didn't have to expend much physical effort during this project. So I could scrunch under an overhang and feel sorry for myself.
The problem with fieldwork is that sometimes it takes a lot of money/coordination to get everything set up, and you may have a small window of opportunity to get everything done. It also may be impossible to get someone to cover for you. So most of the time, I just suck it up if I'm sick.
I've only stopped fieldwork because of illness once. In that case, one of the subcontractors was also fighting the same bug, so when I gave up mid-morning, the response was "yay!" and they tore out of there as if they were afraid I would change my mind.
So how much misery does it take you before you give up and go home?