Giardia is a common parasite that infects the digestive tract of humans and animals worldwide. It usually comes from food or water that's been contaminated by infected stool and is especially prevalent in developing countries. In the United States, giardia is most common in mountainous regions.
The condition caused by giardia infection is called giardiasis and includes such symptoms as nausea, vomiting, bloating, diarrhea and gas cramps. Some cases are more severe, while some are more mild.
One 2011 study suggests that having giardiasis may raise your risk of later developing chronic fatigue syndrome or irritable bowel syndrome. However, we don't yet have enough evidence to confirm the link or to say whether giardia causes or contributes to these illnesses.