Here's a quote from an article by Dr. Kelly A. Reynolds, a University of Arizona Research Scientist and recognized authority on pathogenic agents in drinking water:
“Conventional chlorine disinfection of public water supplies is not an effective barrier for human protozoan pathogens, such as Cryptosporidium and Giardia, which are major waterborne pathogens. Ultraviolet light irradiation (UV) can effectively inactivate these hard to kill protozoa, without producing harmful disinfection by products.”
—Water Conditioning and Purification, Sept. 2004, p. 94.
(whole house & well units)