Excystation of Cryptosporidium parvum. Cysts of this water-borne parasite can be found in some public water supplies. Takes only 4-6 cysts to infect. You drink the water, the cyst passes the stomach where acid weakens the cyst wall, and in the intestine, the little caraway seed-like sporozooites invade the intestinal wall and produce more cysts. Sporozooites are shown escaping their cysts. Can be fatal in suppressed immune individuals like AIDS.