Swimming pools and hot tubs are well known centers for the exchange of bodily fluids since it is a setting in which people regularly consume the fecal matter of others. Though public pools routinely evacuate swimmers when visible feces are present, non-visible fecal matter is basically omnipresent.
The recent alerts regarding cases of Cryptosporidium infections in recreational swimming facilities is such an example of occult fecal ingestion. Cryptosporidium is a protozoal organism that is basically found everywhere and is infamous for a 400,000+ person outbreak it caused in 1990s Milwaukee. Infection occurs when someone ingests the oocysts of the organism that then hatch in the intestine. Though usually a benign illness, it can be particularly severe in the immunocompromised or in the malnourished.
Cases of Cryptosporidium can sometimes produce little to no symptoms but still be contagious, a factor that may be responsible for some infections. Also, the organism's ability to tolerate chlorine makes it particularly successful.
So bottom line: don't drink the water in pools or hot tubs and be wary of pools with diaper-wearing creatures present as they are always leaking some sort of fecal matter.