Mental Floss

What Happens When a Fly Lands on Your Food?

How gross does it get once that fly takes a swan dive into your bowl of soup? You probably don’t want to know the answer.
ByKate Horowitz|
Not quite a Brundlefly, but still too close for comfort.
Not quite a Brundlefly, but still too close for comfort. | Oxford Scientific, The Image Bank, Getty Images; Justin Dodd, Mental Floss (background)

When it comes to food, it’s easy to be picky. But the question of whether to dig in and devour a meal gets even more complicated when you think about bugs—and specifically, flies—ending up within that otherwise delicious-looking grub. Say you’re eating a bowl of soup when one takes a swan dive right into it—is the soup still safe to eat? If you really want to know, gird yourself and read on.

  1. What Really Happens When Flies Land in Food?
  2. What Are the Potential Risks?
  3. So, Should You Toss Your Food Out?

What Really Happens When Flies Land in Food?

The common house fly (Musca domestica) has no venom, no stinger, and no fangs. It finds its food in a peaceful way—by rolling around in the waste and garbage of other animals. With no teeth, the fly requires a liquid diet.

This would be a problem, since a lot of food is solid, but flies have a disgusting workaround: Each one spits and pukes on its meal. Compounds in its saliva and bile break down the food, making it as slurpable as a smoothie.

As the fly eats, it’s usually also pooping—and if it’s female, possibly laying eggs as well. Flies really are an absolute bonanza of disgustingness.

What Are the Potential Risks?

All of this would be gross, but ultimately harmless, if flies only ate soup. But they’re opportunists. They eat rotting garbage and they eat animal feces, and in doing so they consume loads of pathogens.

“House flies are the movers of any disgusting pathogenic microorganism you can think of,” Jeff Scott, an entomologist at Cornell University, told the Daily Mail. “Anything that comes out of an animal, such as bacteria and viruses, house flies can take from that waste and deposit on your sandwich.”

Experts estimate that adult houseflies can transmit more than 100 different diseases and parasites, from Salmonella and tuberculosis to tapeworms.

So, Should You Toss Your Food Out?

Does this mean we should immediately throw out any food a fly has touched? Probably. According to recent research, houseflies tend to harbor pathogens on their legs, meaning even a brief touchdown on your tuna melt could conceivably transmit any number of worrisome bacteria in an instant.

It’s better to keep picnic foods sealed until they’re needed or to ask for a new dish if your local restaurant has any unsolicited, winged guests lurking around. 

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A version of this story was published in 2017 and has been updated for 2025.

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