Travelers' diarrhea (TD) can be caused by viruses and parasites, but is most often caused by bacteria.
People get traveler's diarrhea by eating food and drinking water that contain germs. People can get this illness in areas of the world where the drinking water is not clean.
When people travel to a new place, they are more likely to become sick, because they lack protective antibodies (infection-fighting agents in the blood) that attack these germs.