A potent germicide, UV-C light has this property. It renders harmless waterborne microorganisms that are fully unaffected by chlorine-based disinfectants and deactivates them. Giardia lamblia and Cryptosporidium parvum are two deadly pathogens (viruses and cysts) that can be destroyed by UV-C light and are unaffected by chlorine. The papillomavirus, e-coli, and pseudomonas are only a few of the bacteria that UV-C is effective against. To ensure that the water in the swimming pool is not only disinfectant but also disinfection, a secondary disinfectant must be added to the water. Chemicals with a chlorine basis are the most widely used secondary disinfectants.