Giardiasis is an illness caused by a parasite called Giardia intestinalis. It lives in soil, food, and water. It may also be on surfaces that have been contaminated with waste. You can become infected if you swallow the parasite. You can also get it if you're exposed to human feces (poop) through sexual contact.The risk of getting giardia is higher for travelers to countries where it is common, people in child care settings, and those who drink untreated water.Diarrhea is the main symptom of giardia infection. Others include Passing gas Greasy stools Stomach cramps Upset stomach or nausea These symptoms may lead to weight loss and loss of body fluids. Some people have no symptoms at all.