1. Eggs (oocysts) are excreted by the infected host in stool and possibly in respiratory secretions. The eggs can cause infection (are infective) when excreted.
2–3 (a–k). The eggs are swallowed in contaminated water or food (or possibly inhaled) and hatch inside the body. The parasites change forms several times. They multiply asexually first. Then the parasites develop into a male form (microgamont) and a female form (macrogamont). These forms mate and produce eggs. Two types of eggs are produced: thick-walled eggs, which are commonly excreted by the infected person, and thin-walled eggs, which are involved primarily in infection of self (autoinfection). In autoinfection, eggs develop into adult forms and cause infection without ever leaving the body.
Cryptosporidium parasites are transmitted mainly through contact with contaminated water (for example, in drinking water or swimming pools) and occasionally through contaminated food.
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.