Determining that you got sick from impure water can be difficult. You might try to run by external symptoms only, but that is far from foolproof. As was stated, several other trailwise maladies look about the same.
I was trekking in Nepal, and our Sherpa cook was supposedly boiling all of our water. Somebody might have slipped up on that. One woman trekker was getting bad symptoms one day. Her bowels were getting so loose that she had to get off the trail into the bushes a dozen times. That gets extra bad when your elevation is too high for bushes.
That afternoon, we reached our campsite at Pheriche (about 14,000'), and we went to the trekker's aid post to have afternoon tea with the two western doctors who worked there treating trekkers and locals. Since the woman trekker was so concerned, she asked one of the doctors about her symptoms. One doctor stated that the standard symptoms of Giardia lamblia might be this or that, but the only way to be sure is by a microscopic examination of a stool specimen. The woman responded that they probably won't need to wait long for a new specimen.
A few minutes later the new specimen was under the microscope, but the doctor stated that it might take 15 minutes of looking before the necessary count of cysts was reached. Within seconds, the doctor had the count. It was quite obvious, because there were millions. This was an extreme case.
An hour later she had started taking drugs to kill the cysts, and she made a full recovery after some days.
How many of you Through-Hikers are carrying a microscope along?
–B.G.–

