In most cases, waterborne pathogens use an animal or human body as a host, whereby they rapidly reproduce. A person may ingest only a couple of microorganisms, but after getting sick, could defecate several million. The largest of the waterborne pathogens, protozoans are the easiest of the three to remove from water by filtration, but some can be difficult to kill via iodine, chlorine or the like because of a protective shell that resists chemical water treatments. The two most common and publicized protozoans are Giardia lamblia and Cryptosporidium. Probably the most prevalent parasite in American waterways, giardia cysts range in size from 8 to 12 microns but have been known to squeeze through holes that are 5 to 6 microns in diameter.
Giardiasis is contracted by drinking water that an infected animal or human has used as a toilet.
The disease is characterized by a weeklong bout of diarrhea, which usually manifests five to seven days after ingestion, along with possible bouts of abdominal cramps, fatigue and weight loss. Although most mammals can carry giardia cysts, the primary offenders are beavers and muskrats because they spend most of their time in and around the water. Floating along in cool water, a giardia cyst can survive up to three months, and consuming as few as one to 10 cysts can make a person ill. Cryptosporidium is slightly smaller than giardia, able to pass through a 3-micron hole. After ingestion, it takes two to four days for crypto to make you ill, and the symptoms of cryptosporidiosis are almost identical to giardiasis.