Coliform bacteria are organisms that are present in the environment and in the faeces (poo) of all warm-blooded animals and humans. Coliform bacteria are not likely to cause any illness. However, their presence in water indicates that disease-causing organisms (pathogens) could be in the water. Most pathogens that can contaminate water supplies come from the faeces of humans or animals. Testing water for all possible pathogens is complex, time-consuming, and expensive. It is relatively easy and inexpensive to test for coliform bacteria.