The protozoan Cryptosporidium is notorious for its resistance to chlorine disinfection, a mainstay of water treatment. Human infections, mainly of the small intestine, arise from consumption of faecally contaminated food or water, environmental exposure, and person-to-person or animal-to-person spread. Acute gastrointestinal symptoms can be prolonged but are usually self-limiting. Problems arise with immune-deficient, including malnourished, people including chronic diarrhoea, hepato-biliary tree and extra-gastrointestinal site infection, and few options for treatment or prevention exist. Although genomics has enabled refined classification, identification of chemotherapeutic targets and vaccine candidates, and putative factors for host adaption and pathogenesis, their confirmation has been hampered by a lack of biological tools.