C. parvum and C. hominis LIFE CYCLE
Crypto begins its life cycle as sporulated oocysts (1) which enter the environment through the feces of the infected host. There is some evidence that it can also be spread by respiratory secretions.
The infective oocysts reside in food and water (2). Most transmission occurs through recreational water use, such as in pools and lakes. Crypto has also entered drinking water supplies, causing major outbreaks in cities around the world. Oocysts are resistant to chlorine treatment, presenting a major water purification problem during outbreaks. They can also withstand freezing temperatures.
Infection occurs when the oocysts are ingested by a suitable host (3). While in the intestines, the oocyst releases sporozoites which invade the epithelial linings of the intestines or the lungs (depending on the method of transmission). Within these cells, the sporozoites undergo schizogony, or asexual reproduction. The sporozoites then enter a sexual reproductive stage. Female macrogamonts and male microgamonts develop, and fertilization occurs. The resulting zygote can develop into: a) a thick-walled oocyst that will exit the host, or b) a thin-walled oocyst which will autoinfect the host.
The infectious oocysts are passed through the feces (1). These oocysts enter the environment to await ingestion by the next host.
From Laboratory Identification of Parasites of Public Health Concern, CDC. (http://www.dpd.cdc.gov/dpdx/HTML/Cryptosporidiosis.htm).