What happens if you eat food that flies landed on?

Answerness

Not much really.

Flies will be probably digest the food is has landed on, and to do that they need to digest it. However flies have a poor internal digestion system so they will 'vomit' digestive fluids on the fluid mix the food and the fluid and then consume it.

This is quite disgusting but the fluid is weak and you can still digest the food easily without knowledge of what has transpired.

If however, the food has been visited by many flies for a long time, the digestive fluids may be able to cause a few little problems for your digestion.

Also flies act like transport vehicles to many diseases, so if it lands on something it may have unknowingly left a small present of bacteria for you. Thankfully the bacteria will be in too small numbers to cause a problem if a fly landed on it. But you should really think how much you want to eat if lots of flies land on it, as too much bacteria can cause problems with digestion and could give you a cold (or in very rare cases, a very bad disease). Fortunately, the bacteria will fail to medicine like penicilin.

If one or two flies land on food, it should be safe and with any luck it will build up your immunity.