I was infected with Giardia once (source unknown but I suspect a glass of water at a truckstop in PA) and waited a month before going in to see a doctor. The first two or three days were constant trips to the bathroom about every hour or so, although for some reason I only needed to get up once per night while sleeping. After that it seemed to level off and although my stools remained loose to very loose, one or two sessions on the throne per day were all I needed.

The doctor that tested and then treated me tried to give me a stern lecture about how people die from waiting so long before coming in for examinations when they have problems. I just asked him if, in his expert medical opinion after examining me, he thought I was in any danger of kicking the bucket in the next few days. He got mad at me! lol

I suspect that people in the past were infected with lots of stuff but just accepted it as part of living, adjusted accordingly and went on with their lives.