*worldwide
*2nd important pathogen for moderate-severe diarrhea before 1 year of life
*repeated outbreaks even in developed countries with huge number of cases
**human excretes 100 mil. cysts per day
**calf excretes 10 bil. cysts per day
**water chlorination does not prevent tap water borne infection
===life cycle===
*survive >1 year in 4-6℃ cold water
*vulnerable to dry environment
*human excretes 100 mil. cysts per day
*calf excretes 10 bil. cysts per day
===morphology===