Cryptosporidiosis is a disease caused by unicellular parasites belonging to the Cryptosporidium genus. The small intestine is the primary site of localization of infection which predicts the main clinical symptom of the disease — diarrhea. The most important factors influencing Cryptosporidium infection and the course of disease are molecular genetic variability of the parasite, its virulence and infectivity, and viability of the mucosa of the digestive tract and local and systemic immunity of the macroorganism. The immune status of the host plays a key role in determining sensitivity to infection and the severity of the disease. Cryptosporidium infection differs in outcomes: asymptomatic in some patients, acute enteritis accompanied by profuse diarrhea, lesions of internal organs, and fatal outcome in others. Current therapeutic approaches to the treatment of cryptosporidiosis are ineffective. Despite the existence of a large number of drugs with antiparasitic effect, there are no medications with a specific effect on cryptosporidia. Understanding the factors that determine both the pathogenicity of Cryptosporidia and the protective properties of host defense systems will allow developing effective prevention measures and therapeutic interventions of this protozoosis.