Many disease-causing organisms are found on flies. They carry these disease on their outer body (within tiny hairs), in their gut and in their blood stream. They acquire these diseases from the habitat they live in and around i.e. decaying organic matter, faeces and infected or contaminated material in which they breed. These diseases are then deposited on to food, food prep surfaces, cooking utensils and crockery. The diseases deposited by flies can cause and are related to many acute gastro/gut complaints in humans. Most recently there's evidence to suggest that the transfer of E.coli is by flying insects.