Parasites
No one wants to think that stomach symptoms are due to a parasitic worm or other creature. But it happens. Parasites can attach themselves to your stomach lining or hang out in the small intestine or colon, leading to a host of unwanted symptoms.
There are many types, but the most common in the U.S. are Giardia and Cryptosporidium, which you can get by swimming in contaminated pools or lakes or drinking contaminated water. (Or in some outbreaks, unpasteurized cider). The tiny protozoa cause cramps, diarrhea, and nausea about 2 to 10 days after exposure (for Crypto) or 1 to 3 weeks later (for Giardia). Other types of parasites can be picked up in undercooked or contaminated food. Dr. Raymond says sushi, when obtained from a less than reputable source, could contain parasites.
