**1. Introduction**

*Cryptosporidium* species are protozoan parasites that infect the epithelial cells of the gastrointestinal and respiratory tracts of humans and a wide range of animals, with a global distribution [1–3]. *Cryptosporidium* represents a major public health concern for waterborne disease and daycare outbreaks of diarrhoeal disease worldwide [2, 4–8]. Human cryptosporidiosis is usually a self-limiting infection in immunocompetent individuals. However, cases of severe diarrhoea and dissemination to extra-intestinal sites can occur in children, the elderly, and individuals with impairment of T-cell functions, mainly those with HIV infection [9–12]. In children, although diarrhoea is a key feature of malabsorption, it may not be apparent at presentation; when the infection becomes chronic, the only symptom may be limited growth. Consequently, chronic infections can culminate in poor growth [5, 13–16]. The epidemiology of infections is complex and involves transmission by a faecal-oral route, either by ingestion of contaminated water or food or by human-to-human or animal-to-human transmission [17, 18]. The oocyst, the environmental stage of *Cryptosporidium,* is incredibly hardy, easily spread through water, and resistant

to inactivation by chlorine; and without the use of filtration, it is challenging to remove it from drinking water [19–21]. *Cryptosporidium* prevalence is higher in areas lacking a sanitation infrastructure, mainly drinking water and sewage, which led the World Health Organization (WHO) to include it in the water sanitation and health programme [22]. The scarcity of sustained access to safe water creates tremendous burdens of *Cryptosporidium* diarrhoea in developing countries [23]. Treatment and diagnosis options are still not totally effective [2, 24–26]. No fully effective drug therapy or vaccine is available for *Cryptosporidium*, and the diagnosis of cryptosporidiosis has been based on the demonstration of oocysts in faeces, which present low sensibility [25]. However, the ability to culture relevant *Cryptosporidium* isolates in vitro, the development of novel gene-editing tools (knockout genes, CRISPR/ Cas9, and RNAi) [26–30], and 'omic' research (genomics, transcriptomics, and proteomics) represent essential paths towards significant advancements in the control of cryptosporidiosis [30–38]. In the future, those approaches will show a holistic view of the biology of *Cryptosporidium*. In this chapter, we present recent advances and remaining challenges regarding human cryptosporidiosis under a public health perspective.
