Hi Beth-thanks for the interesting question! I'm not aware of a lot of research in this area, but here are some things to consider. First off, there's no way to guarantee the safety of the leftover food you would be eating; simply put, anything that touched someone's mouth was exposed to all the bacteria that live there--some of which are potentially pathogenic, or disease-causing. Additionally, pathogens live on the hands and can be spread to food by poor hygienic practices. You brought up hepatitis-there are several types of hepatitis, but only Hepatitis A (or E, although this would be less common) would be a concern; this can be transmitted orally. If you heat the food to at least 185 degrees C for at least a minute it theoretically should kill the virus. Oral herpes is a virus that can be transmitted through contaminated food and drink. And, there are a host of other pathogenic bacteria that could be spread through food. All this being said, lots of people do what you're thinking about. If you decide to try it, I'd suggest avoiding altogether any type of food that's required multiple passes with a person's fork or mouth, such as mashed potatoes or a chicken wing. Stick to something that can be separated into an 'exposed' portion and an 'unexposed' portion, like a steak filet that is only partially eaten (you would cut off the end that has been eaten from and eat the remaining portion). And avoid anything that has been near anyone with any signs of illness (cough, sneeze etc). For good measure, it might be a good idea to avoid eating food from patrons that got up during their meal, just in case they used the restroom and failed to wash their hands.