Infection of humans occurs by the fecal-oral route or via ingestion of contaminated water or food. Infection is easily acquired, with an infectious dose of approximately 100 oocysts. Waterborne transmission is a primary mode of transmission and is commonly responsible for human outbreaks. This is due to the fact that Cryptosporidium species oocysts are resistant to cold temperatures and chlorine and require extensive filtration or water treatment to remove them from drinking water.