Cryptosporidium

Cryptosporidium is a genus of parasitic protozoans that infect the intestinal lining of humans and a wide range of other vertebrates, transmitted through water or food contaminated with its resistant, thick-walled oocysts. It is a leading cause of waterborne disease outbreaks worldwide because its oocysts are unusually resistant to standard chlorine disinfection used in municipal water treatment, causing cryptosporidiosis, a diarrheal illness that is typically self-limiting in healthy people but can become severe and life-threatening in those with weakened immune systems.