Cryptosporidium parvum, a single-celled animal, i.e., protozoa, is an obligate intracellular parasite. It has been given additional species names when isolated from different hosts. It is currently thought that the form infecting humans is the same species that causes disease in young calves. Cryptosporidium infects many herd animals (cows, goats, and sheep among domesticated animals). The infective stage of the organism, the oocyst is about half the size of a red blood cell. The sporocysts are resistant to most chemical disinfectants, but are susceptible to drying and the ultraviolet portion of sunlight. Some strains appear to be adapted to certain hosts but cross-strain infectivity occurs and may or may not be associated with illness.
Cryptosporidium is a highly infectious and potentially dangerous parasite, especially when confronted by a weak immune system. When it attaches to the lining of the small bowel, it then multiplies over and over again and attacks the intestine, as a response the body tries to flush it out, this reaction causes diarrhoea. While some is washed away the rest is multiplied, lining the stomach walls. By lining the stomach walls, Cryptosporidium prevents the body from absorbing nutrients out of the body, eventually causes malnutrition and death.
Part of the reason Cryptosporidium is so successful at attacking humans, is because it has a great strategy, it uses water to get to its host, by using water Cryptosporidium is able to affect large populations very quickly, and it is able to spread over large geographical areas. Chlorine kills most stuff in the water, but not even chlorine kills this nasty protozoan. Cryptosporidium has a protected coating which allows it to withstand chlorine based cleaning systems. Ozone treatment reduces the amount of Cryptosporidium in drinking water, but it does not remove all of it. On average there are ten parasites in every 10 gallons of drinking water in Ireland. This concentration rarely causes illness.