Below is an illustration of the life cycle of C. parvum from the CDC , both within and outside the human host. It depicts the most common form of transmission, from contaminated water, although it may also be transmitted from other sources. Click here for examples of other modes of transmission.
Sporulated oocysts with 4 sporozites each are excreted by the infected host (either human or animal) through feces and possibly other routes such as respiratory secretions 
. Cryptosporidium parvum, as stated before, is generally transmitted through contact with contaminated drinking or recreational water 
. After ingesting, or possibly inhaling the parasite 
, excystation 
occurs when the sporozoites leave the oocyte. The sporozoites are released and infect epithelial cells (
, 
) of the gastrointestinal tract or other tissues such as the respiratory tract. In these cells, the parasites undergo asexual multiplication, a.k.a. schizogony or merogony, (
, 
, 
) and then sexual multiplication, or gametogony, producing microgamonts (males) 
and macrogamonts (females) 
. When the macrogamonts is fertilized by the microgametes (
), oocysts (
, 
) develop that sporulate in the infected host. Two different types of oocysts are produced, the thick-walled, which is commonly excreted from the host , and the thin-walled oocyst , which is primarily involved in autoinfection. The thick-walled, excreted oocyst is particularly difficult to kill, making chlorine an insufficient agent to decontaminated water. Oocysts are infective upon excretion, thus permitting direct and immediate fecal-oral transmission.
There are no vectors for Cryptosporidiosis. However, there are multiple mammalian reservoirs including cattle, dogs, and other domestic animals. Once inside a human, the incubation period for C. parvum may be anywhere from 2-10, but averaging a week. Because Crypto is highly contagious, it is important to diagnose it as early as possible.