Since pioneers such as John Snow and Robert Koch first established water as major route of disease transmission in the late 19th century, many countries have attained a high level of drinking-water safety. Nonetheless, these continue to struggle with some disease agents found in water, due to new pathogens evolving or being discovered, new chemicals being developed and found in water, and due to the development of our scientific understanding of both with respect to their health impact and their persistence in the water environment.