There are at least a hundred different bacteria and viruses that a fly can transmit when it falls on food, says Thomas J. Daniels, scientist and director of the environmental lab at Fordham University in New York.
According to The Healthy magazine, when a fly catches bacteria, it can transmit them around as soon as it lands, even on food, despite being just above it for a few seconds. “There is a risk of infection and disease, despite the fact that the amount of bacteria transmitted in that short time is small,” says Daniels.
However, a greater risk comes when the fly “vomits”, ie when it swallows food. When it hits food, the fly does not actually eat it immediately, but releases fluids that will soften it, so that it can “eat” it in a more liquid form. The body fluid that flies release into food is full of bacteria, so the chances of becoming infected are much higher if the food is “spit” than when it just landed on the food.
The fluids released by the fly are more dangerous because they come from her body, while the bacteria found in her body are less dangerous. Therefore, if only one fly lands on the food, you do not have to throw it away immediately. You are more likely to get infected if more flies are sitting, which usually happens when you eat outside.