Cryptosporidium is a genus of waterborne pathogenic protozoan parasites that cause a severe gastrointestinal disease called cryptosporidiosis . Both humans and animals can be infected, and of particular note are the species that commonly infect humans, Cryptosporidium parvum (C. parvum) and Cryptosporidium hominis (C. hominis) . Infection can happen through person-to-person contact, animal-to-person contact, or via ingestion of contaminated food or water . Cryptosporidiosis is one of the most common waterborne diseases and one of the common causes of death among young children (48 000 cases in 2016) . Outbreaks occur globally, resulting in >12 million disability-adjusted life-years with high costs for society . One of Cryptosporidium's strengths as a pathogen is its ability to form resilient oocysts, which are 5 m in diameter spherical particles. See SEM images of C. parvum oocysts in Figure and Figure . Oocysts are non-motile and can remain infective outside the host for over one year . Each oocyst contains four sporozoites that infect the epithelial cells in the intestine and ingesting as few as ten oocysts can be sufficient to cause infection in humans .