Giardia

Giardia thrives within intestines of infected humans or animals. One becomes infected through ingesting or coming into contact with contaminated water, food or soil. The Giardia parasite spreads when a person accidently ingests it, this can originate from contaminated items and surfaces that have been tainted by the feces of an infected human or animal. The parasite can also transfer from consuming unsanitary food or water
The symptoms of Giardia include: diarrhea, excess gas, stomach or abdominal cramps, and upset stomach or nausea. This can result in weight loss or dehydration, which can be harmful if not treated immediately. Typical infection within an individual is around 2-6 weeks, but medicine can shorten this time. In rare cases giardia can become a chronic disease.
http://www.webmd.com/a-to-z-guides/giardiasis-topic-overview