Why are pathogenic microogranisms dangerous? When contaminated water is consumed, pathogens can enter the digestive system and cause illnesses such as cholera, dysentery, gastroenteritis, and cryptosporidiosis. These diseases often cause vomiting, diarrhoea, dehydration, fever, and stomach pain. In severe cases, especially in young children, elderly people, or people with weakened immune systems, these illnesses can become life-threatening. This is why monitoring microorganisms is one of the most important parts of water treatment.