Cryptosporiodiosis

Cryptosporiodiosis is a disease affecting the intestines of mammals that is caused by Cryptosporidium, a protozoan parasite in the phylum Apicomplexa. It is spread through the faecal-oral route.

Symptoms appear from two to ten days after infection and last for up to two weeks or so. As well as watery diarrhoea there is often stomach pains or cramps and a low fever. Some individuals are asymptomatic but are still infective and thus can pass on the infection to others. Even after symptoms have finally subsided that individual is still infective for some weeks.
 

Despite not being identified until 1976 it is one of the most common waterborne diseases and is found worldwide.

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Sources:
http://en.wikipedia.org/wiki/Cryptosporiodiosis
  Picture:
  http://www.mwra.state.ma.us/germs/crypto.htm