There’s a Fly in My Soup!?>
Ask the Experts

There’s a Fly in My Soup!

by Laurie Saloman  

Q. Can I get sick from eating food that a fly has landed on?

A. It’s unknown. Studies have shown that houseflies and blowflies, also known as common green bottle flies, may be carriers of more than 200 different infectious agents. Why? Flies eat and lay their eggs in feces, rotting flesh, and other decaying substances, where they come into contact with a variety of pathogens (disease-causing microorganisms). These pathogens stick to different body parts as the flies are feeding and reproducing.

Frequent back-and-forth travel between environments where humans spend time and the piles of feces and other waste in which flies thrive enables them to potentially transmit disease. Besides carrying pathogens on their bodies, flies are known to feed by literally regurgitating enzymes from their bodies onto food. The enzymes break down the food into a liquid that the fly slurps up. As a result, any pathogens inside the fly can end up on your food.

By now, you might understandably conclude that a fly landing on your food might be a good reason to toss it—but that’s not necessarily true. While researchers can prove that flies carry diseases that pose a health threat, most of them stop short of saying conclusively that flies can transmit diseases to healthy people. That’s because there’s no hard evidence that flies transmit pathogens through food that people consume.

On the other hand, some researchers hypothesize that a growing fly population could be associated with equally increasing bouts of diarrhea. So the risk of infection from microbes on food that are transmittedby one fly is low—although if a swarm of flies is crawling on your food, you certainly shouldn’t eat it.

And, if you’re immunocompromised for any reason, you might want to err on the side of caution and pass on any food that flies have landed on, particularly if the insects been there for more than a few seconds.

This article first appeared in the April 2020 issue of UC Berkeley Health After 50.

Also see Can Your Dog or Cat Make You Sick?