The organism is called Cryptosporidium parvum, and it does live in water.
This is not a bacterium or mould, but it is a free-living parasite.
According to the American Society of Microbiology's Manual of Clinical
Microbiology, 5th ed., the organism causes diarrhea and may transmitted by
person-to-person contact or may be waterborne. The disease, called
"Cryptosporidiosis" may attack anyone, but may be particularly severe in
immunocompromised patients, such at AIDS victims or cancer patients who
have suppressed immune systems from chemotherapy.
I hope this helps; but I don't know anything about therapy to control the
organism. Perhaps Flagyl?