Not too badly, but yes, I've been sick on a trip... happened just a couple weekends ago on a backpacking trip... probably over-exertion, maybe a food bug... don't know... First overnight was fine, at the trailhead, but might have had some sketchy bbq chicken... next day, I was fine for a 3 mile hike with pack, got my camp set up, and then suddenly felt queasy and had a slight headache... thought it was dehydration, so I pushed fluids for an hour... about a quart or so. Temps were only in the 40s/50s, so I wasn't sweating much. Regardless, I lost my lunch, mostly water (which tells me it wasn't the chicken), slept for 2 hours, woke up weak as a newborn chick, but felt much better... helped my buddies drag in a little more firewood, ate dinner, and went to bed all as normal... woke up the next morning just fine.
I don't carry a very extensive first aid kit, but one thing I do carry is a pain reliever and some pepto... I didn't use them, but what happened to me could have been much worse, and I could have dehydrated very easily.