Among the apicomplexan parasites of medical interest, Cryptosporidium is the least studied. Consistent with other members of this phylum, the generally accepted life cycle of Cryptosporidium comprises a sexual phase during which gamete differentiation and fertilization occurs. This process can affect the population structure of this parasite because of the possibility of genetic recombination between genotypes. Experimental evidence has confirmed that recombinant genotypes can emerge from mixed infections. A recent study of Cryptosporidium parvum populations infecting humans and livestock in Aberdeenshire, UK, shows that recombination also takes place in nature. Distinct population structures were identified among C. parvum types (species) infecting humans and livestock.