Fortunately it only happened to me once. It was on a St. John River (Maine) trip. It was just after one of the finer memories from that trip: My 11-year old bow woman and I had garnered cheers from a group that was scouting the class II Big Black rapid. We had started down the left side, then pulled off a cross-river ferry in a heavily loaded canoe, then peeled and snaked down the remainder of the rapid on the right. We'd practiced these moves on the first few days of the trip and it was uplifting to see Katie reacting to what she saw in front of her before I could shout instructions. I'd think, "she needs a cross-draw here," and she'd be doing it before I could get the words out of my mouth. It must have looked impressive from the banks because the group that was scouting was hooping and cheering us on. Anyhow, we get to the bottom of the rapid and I was elated until all of a sudden, I didn't feel good. Fifteen minutes later I was hurling over the gunwales.
We made camp a short distance below the rapid. I was sweating and had fever chills and it was an effort to get my gear up the bank, set up my tent, and crawl in. I went to sleep almost immediately. I came out of the tent once to puke and then went back to bed. I was pretty bummed thinking getting sick was really going to screw up the trip, but in the morning, I was fine.
One never knows for sure, but I suspect I was a victim of inadequate camp hygiene. I try to be careful, but it is easy to slip up, especially traveling with kids. Whatever bug knocked me down, I was thankful it was fast moving.