Fruit flies do not bite humans but are they still dangerous? You may have seen red have a fruit fly problem. Did you wake up in the morning with the marks? Fruit flies generally prefer foods that are rotten, decaying or just things that are generally not healthy. This can cause them to carry dangerous bacteria that are Missing: breakfast. You don't just need to know how to kill fruit flies, you need to prevent them These all-natural methods will get you fly-free fast! Change every morning. I leave a bowl of fruit on the kitchen counter to keep healthy snacks within . or brown or grey files, that bite, and can get through almost any screen.