How many protozoa are in the water we drink? Researchers have analyzed drinking water and detected oocysts of Cryptosporidium and cysts of Giardia, two protozoa that cause outbreaks of diarrhea in humans. The levels detected are very low and do not represent a health risk; however, according to the study, the ubiquity of these parasites and the inefficiency of conventional water treatment in reducing them may present a public health issue. Source 9d.