Travelers are cautioned against drinking any untreated water. It causes infection up to 20 percent of the world's population, and is most prevalent in developing countries, where infections are associated with poor sanitary conditions, poor water quality control, and overcrowding. It occurs primarily in areas where water supplies have become contaminated with feces from humans or, possibly, from animals. Outbreaks have occurred throughout the country. Infection begins when the cysts are consumed through food or water.
Stomach acid activates the cysts and the trophozoites are released.
They become attached to the lining of the small intestine and reproduce. Cysts form in the lower intestines and are then passed in the feces. The parasite may be directly transmitted from person to person by contact with infected feces, or indirectly, through consuming food and water contaminated with cyst-containing feces. The time between infection and the onset of acute symptoms usually is from one to two weeks. Giardiasis cannot be prevented, but the spread of it can be contained by practicing good personal hygiene, as well as proper hygiene when caring for those who may be infected with the parasite.