Giardiasis is a disease caused by the ciliate protozoan Giardia lamblia. It is a common cause of diarrhea.

Giardiasis is passed via the fecal-oral route. People who live in institutional or day-care environments are more suspetible, as are travelers and those who consume improperly treated water. It is a particular danger to people hiking or backpacking in wilderness areas worldwide.

Treatment is not always necessary, as the body can defeat the infection by itself. Antibiotics used in adults include metronidazole and albenzazole. (Furazolidone and nitazoxanide may be used in children)