It's really gross what happens to food when a fly stands on it.
From the moment a fly lands on our food plate, these can leave dangerous bacteria in our food, such as salmonella or E. coli, pathogens that they carry with them, as we know that flies land on any unpleasant waste.
Did you know that flies do not have teeth? However, in order to dissolve the food they must vomit a little of their saliva to be able to possibly taste our forgotten lunch discovered on top of the kitchen. So, when posing, it leaves its dangerous bacteria and also they vomit the food and of course they also defecate it.
Do I throw food away when flies are on my plate?
When a fly lands on something, it leaves parasites and bacteria, because by its nature they always carry it on their legs. But, if when you saw that a fly barely landed on your food and you immediately scared it away, then it is very likely that this does not cause you serious problems, since a healthy person with her perfect immune system could counteract that.
However, that is only if it was only a short time after the fly was chased away, because if the food was exposed to flies for a long time, then it is when the pathogens multiply and the risks for diseases are more serious.
Never trust the uncovered food, much less if there are flies hanging around the place, you will never know if they have been posing on your food for a long time or worse, you will not know if the time you were not present they could vomit and defecate on the food.