Even though flies are a nuisance, they are also dangerous. There are many health concerns associated with flies, especially because of the substances they feed on. They can land on feces and then a human, with nobody the wiser. These flies can carry more than 100 pathogens, which can cause diseases like cholera and tuberculosis. Not only can a fly spread a disease simply by landing on you, but they can also spread dangerous pathogens through their vomit, feces and hair.