Carry at least one liter, esp if hiking alone. I found out in January how fast illness can strike. While hiking in Big Bend National Park (Mesa de Anguila), the temps were in the 40's or 50's and steadily drizzling. In the morning I was fine. We had lunch on a small ridge. I do remember feeling slightly anxious about only having my driducks with me since we would be desert bushwacking (think cat claw). I was not cold...just cool.
In the early afternoon we were walking through a dry wash and suddenly I felt lightheaded and knew if I stayed vertical I would pass out shortly (been there, done that). I immediately told the trip leader, a 60+ y/o tough-as-nails ex-Israeli military friend, about my issue and I went to lie down and get my feet up. While lying on the gravel I did start to get a little cold so we got out a sleeping pad. I may have had a really mild fever because I did start to shiver. Another person heated up a warm drink for me. There were zero other symptoms at this time so we wondered about mild hypothermia. I didn't think so.
After 45 minutes of resting I felt better, so we needed to make a decision about going back or going on to base camp some 10 miles from immediate help. I decided to push onward because I felt 100 percent better. So we hiked several more hours. About a half mile before camp, about 4:30 PM, the lightheadedness hit me suddenly again along with drownsiness. I just wanted to lie down and sleep in the trail. And I did just lie down right off the trail. The person behind me called for the others to come back. I was down for less than 10 minutes. We made it into camp somewhere after 5 PM and just as the sun was going down. It was no longer drizzling. Others immediately put up their shelters and headed to get water from Tinaja Largo. I only wanted to sleep so I was on all fours not doing anything. After 10-15 minutes someone came over to help me put up my shelter. I went straight to bed at 6 PM with a wiped out appetite, weakness, and nausea, but no belly pain that I remember.
Over the next two days, the others dayhiked, but I stayed in camp eating very little the first day and mostly sleeping. I lost hardly any water (no vomiting, one loose bm). My appetite slowly came back over the next two days and by the morning we were to hike out, I was able to eat my breakfast. My strength was about 80 percent by then so I was fine walking out. I also had my appetite back, but made the mistake of eating a greasy hamburger in town. Boy was that a gastronomical mistake. I should have stuck with mild foods for at least 24 hours.
Anyway, that is the long story of how suddenly one can become ill. I was lucky I was with others and had only a mild (to me) viral illness as far as water losses.