After ingestion, it takes two to four days for crypto to make you ill, and the symptoms of cryptosporidiosis are almost identical to giardiasis. Unlike giardia, chemical treatments like iodine or chlorine cannot be used to effectively treat crypto. It must either be boiled or physically filtered out of the water. Bacteria are the next smallest of waterborne pathogens, with some measuring as small as 0.2 micron. Some bacteria are found only in human feces, while others can be present in both domestic and wild animals.
Smaller bacteria require the work of a purifier, rolling boil or chemical treatments.
Viruses are the tiniest microorganisms but are arguably the most problematic. It was traditionally assumed that viruses were not a concern in North America, but recent studies counter that assumption, and today, most experts believe that viruses pose more of a waterborne threat than bacteria. Sporting a diameter as thin as .004 micron, viruses are too small to be effectively removed by filtration, requiring the use of heat, chemicals or a purifier. Also transmitted by the consumption of fecally contaminated water, most viruses typically lead to temporary intestinal problems, but some can cause more serious diseases like hepatitis and meningitis. At this point, the more severe viruses have a very low occurrence rate in North American waters.