There are approximately 1400 human pathogens. That includes bacteria, viruses, protozoa, and helminths that can cause infection. The list of those that can be found in drinking water is not quite as long (see table below). However, the "health significance" as identified by the World Health Organization (WHO) is rated as "high" for almost all of them. Chlorine, which is commonly used to disinfect water supplies, is generally very effective against the bacteria. But when it comes to protozoa, especially Giardia and Cryptosporidium, you may want UV instead!