Cryptosporidium belong to the single-celled parasites, they show low host specificity and in some cases are highly infectious. Oocysts are the inactive, infectious permanent state of the parasite. These are excreted in the feces of infected animals and are very resistant to the outside world. Due to poor hygiene they present a permanent source of infection and therefore a decisive virulence factor of diarrhea. After oral ingestion the Cryptosporidium oocysts encyst in the intestine. They mutate into a vegetative, proliferative shape, the sporozoites, and multiply in the intestinal epithelial cells of the brush border. A heavy infestation of the microvilli with Cryptosporidium can cause malabsorption.
Several Cryptosporidium are pathogenic for the pig. Mono-infections can cause asymptomatic chronic diseases in piglets, which to some extent include high excretion rate of oocysts. In secondary infections with other enteropathogens, e.g. Rotaviruses, a dramatic increase in clinical symptoms and severity of diarrhea can occur.