This presentation is devoted to a discussion of cholera as a paradigm for the study of infectious diseases and then considers the range of infections spread via water before considering how we test water to ensure its microbiological safety. The presentation starts by considering ‘potable’ water, that is, water that is safe to drink. There then follows a list of important pathogens that are spread by exposure to contaminated water. The role of water as the vector of disease was initially described by the epidemiological studies of John Snow, although at the time, his ideas were so revolutionary that they were rejected. His ground-breaking work was then compared with that of another pioneer of infections, Ignaz Semmelweis, who was first to investigate hand-washing as a way of minimising infections following child birth. The second video describes early theories on the spread of infection before considering in detail classic bacterial infections spread by consumption of water: cholera, dysentery and typhoid. The third video begins with a description of campylobacter infection, only recently discovered but now recognised as the commonest cause of gastrointestinal infection in the developed world. Focus then moves to waterborne virus infections, notably infectious hepatitis and poliomyelitis before moving to a consideration of protist diseases , especially amoebic dysentery and cryptosporidium infection , a disease that may be an AIDS defining illness. The fourth video starts with a brief description of amoebic meningitis, caused by Naeglaria fowleri. This infection is unusual in that it is water-borne but symptoms are not associated with the gastrointestinal tract. The history of sanitation in helping to preserve public health is considered net and the presentation concludes with a description of the microbiological testing of drinking water and the importance of faecal indicator organisms. The final remarks highlight how water testing has made an enormous contribution to public health in the developed world whereas in developing countries, waterborne infections still carry a very high risk of death.