Giardia are microscopic pathogens that live in the intestines of several vertebrates including humans. People become infected when they ingest water, food or dirt that has been contaminated by the feces (aka doo-doo, or my grandpa's favorite expression for it, "baddy-baddy") of a host animal or human.
Usually this contamination occurs through unsanitized hands or drinking water. This pathogen gets into the water supply when animals and, unfortunately some people, crap too near or in a stream or other watershed areas.
Incubation time is usually one to two weeks, and can be as much as three weeks, so you will likely (in most cases) be back home when symptoms appear. Symptoms include extreme diarrhea, excess gas (both flatulation and burping), stomach cramps, nausea and vomiting.
Of course, because of the nature of these symptoms, dehydration and weight loss are a serious concern. If you think you have giardiasis (the infection of giardia), you should see your doctor immediately. These little boogers are bad medicine, and if you are infected with these bad boys you may be down from two to six weeks, but your doctor can prescribe medication to decrease this time period of infection.
Prevention can be accomplished by practicing good hygiene, including purifying (this includes the use of iodine water purification tablets or boiling) or filtering your water supply, washing your hands with soap or sanitation wipes, and washing (in purified water) or thoroughly cooking all food items that are not pre-packaged, such as raw fruits and vegetables.