There are at least 100 different pathogens houseflies can carry, like bacteria, viruses, and parasite eggs, says Thomas J. Daniels, PhD, associate research scientist and director of Fordham University’s Vector Ecology Lab at its Louis Calder Center. Yuck!
Once they’ve picked those germs up, the bugs can spread the disease in a few ways. Bacteria and viruses on contaminated food, manure, and other germ-heavy spots stick to flies’ bodies and the tiny hairs on their legs, says Dr. Daniels. Once they fly away and land somewhere else—like your food—they leave some of those germs behind. “So we’re potentially at risk even from a fly landing on food, though the amount of pathogen transmitted is likely to be small,” says Dr. Daniels. (Don’t miss these other habits that are germier than you thought.)
But the bigger concern comes from fly vomit. And yep, it’s as gross as it sounds.
When flies land on your dinner, they aren’t biting tiny nibbles off your plate. Instead, the bugs puke digestive juices onto the food to break it down so they can “lap up a liquid meal,” says Dr. Daniels. But that’s not even the worst part.
The vomit a fly leaves on your food is full of the germs from its last meal. And the pathogens inside the fly live longer than the ones on its feet, which means there’s a bigger chance the bacteria and viruses stayed alive. Those germs mix with the fly vomit and stay in the fly’s mouth until the next time it eats, creating a recipe for disease.
And some of the germs can be pretty scary. Scientists know flies can spread E. coli, salmonella, Hepatitis A, and rotavirus, says board-certified infectious diseases specialist Brent W. Laartz, MD, author of How to Avoid Contagious Diseases. There’s also Shigella—a group of bacteria that can cause diarrhea, fever, and stomach pain—which is particularly concerning because it doesn’t take much for the bacteria to make you sick. (Find out how to tell if your diarrhea could be a sign of irritable bowel syndrome.)
But before you start dumping all the food on your dinner table, know that not every fly will carry all (or any) of those germs. The ones mentioned above are all fecal bacteria and viruses, which don’t exist on everything flies feed on. “The fly would have to land on raw meats or feces to spread these bacteria [and viruses] to your food,” says Dr. Laartz. The cleaner your kitchen, the less chance that the fly picked anything up before munching on your meal. Keep your space clean with these 8 ways to bug-proof your kitchen.
Plus, just one fly on your food isn’t a guarantee that you’ll get sick from its germs. Depending on how many germs the fly carried, how long it sat on your food, and how strong your immune system is, you might never get infected, says Dr. Daniels. So if a single fly touched down once on your food and your house is clean, you can probably keep eating safely, says Dr. Laartz. (But stay extra careful with these 14 tips to avoid getting sick.)
The bigger worry would be if you’re in an area buzzing with bugs, like a picnic. After all, the more flies that land on your meal, the more germs they could leave. “Cover your food while you are waiting to eat it,” says Dr. Laartz, who suggests using a fly cover or another plate. And if you see flies buzzing around food you’ve left uncovered for a while, throwing it out is your safest bet, says Dr. Daniels. Avoid getting sick from what you eat by reading these 7 dangerous foods a food safety expert never eats.