Drinking untreated water -- from a well, stream, or contaminated water system -- can cause diarrhea that may develop within just a few days or up to as much as four weeks after ingestion. It's not the water that's the problem, but the parasites that live in it.
One of the most common parasites found in untreated water is Giardia lamblia, often simply called giardia (gee-ARE-dee-ah). Outdoor enthusiasts often pick up the parasite from drinking stream or lake water during camping or hiking trips.
How can you tell if you're infected with giardia? Your doctor will need to test a sample of your stool. If the results are positive, you may be given an antiparasitic drug (a type of antibiotic), such as tinidazole, metronidazole, nitazoxanide, or paromomycin.