Ultra Violet purification uses a UV light source (lamp) which is enclosed in a protective transparent sleeve (usually quartz). The lamp is mounted such that water passing through a flow chamber is exposed to the UV-C light rays. When harmful microbes are exposed to the UV rays, their nucleic acid absorbs the UV energy, which then scrambles the DNA structure of the organism. The cell is rendered sterile and can no longer reproduce. The cell is now considered dead and is no longer a threat. 

Specifically, the following are moved to an efficiency of greater than 99.99%: E-coli, Salmonella typhl (Typhoid fever), Salmonella enteritidis (Gastroenteritis), Vibrio cholerae (Cholera), Mycobacetrium Tuberculosis (Tuberculosis), Legionella pneumophila (Legionnaires' Disease), Influenza Virus, Polio virus, and Hepatitus A Virus (better than 90%)

UV is known to be highly effective against bacteria, viruses, algae, molds and yeasts, and disease causing oocysts like cryptosporidium and giardia.