UV light (254 nm) damages the generic matter (DNA and RNA) and other crucial molecules inside a microorganism. The primary damage to the microorganisms during ultraviolet irradiation is based on a photosensitive alteration of the nucleic acids that prevents cell division. The damage inactivates the specific micro-organism, because it can no longer carry out vital functions, such as reproduction.
Most bacterial, protozoa, viruses, fungi, and algae are rather sensitive to the exposure of UV-light. However, some species are highly resistant to the irradiation with UV-C light as result of their protective epidermis (skin).