What Actually Happens When A Fly Lands On Your Food?

[00:00:00]A fly just landed on your food. In a second, you'll wave it off without a thought. But, in the moment it's sitting there, it is doing something to your lunch that you would never do to another living person's meal. It is tasting it not with a mouth, but with its feet. It has decided your sandwich is worth eating. And to eat it, a creature with no teeth, no jaw, nothing to bite or chew with, does the only thing it can.

[00:00:21]It throws up on it. That is not a figure of speech. That is the mechanism, and it is only the beginning of what happens in those few seconds, because the same fly that just vomited on your bread is carrying on its body a genetic record of every filthy place it has ever walked.

[00:00:36]By the end of this, you will understand why the flash of disgust you feel is not squeamishness. It is one of the oldest and smartest things your body knows how to do, and someone has already proven it in a lab. Most people think a fly on your food is a nuisance, something between a germ and a joke. Shoo it, keep eating, forget it. That is exactly backwards. The fly is not the problem.

[00:00:55]The fly is a messenger, and the message it leaves behind has been killing people for as long as there have been people, which turns out to be a tiny sliver of the time there have been flies.

[00:01:04]But, there is a twist that almost every video about this gets wrong, and we are coming back to it.

[00:01:09]After everything you are about to see, the honest answer to should I throw the food away? Is not the one you are bracing for. Hold on to that. First, those few seconds. Start with the mouth, or rather the lack of one. A house fly cannot bite. It has no teeth, no jaws, nothing built to break off a piece of solid food. What it has instead is a soft spongy pad at the end of its mouthparts, called the labellum. Picture two fleshy lips folded around a straw, grooved with tiny channels.

[00:01:38]That pad can only take in liquid or particles smaller than half a millimeter.

[00:01:43]Your sandwich is neither. So, the fly solves the problem from the outside in.

[00:01:47]It reaches back into its own gut, pulls up a droplet of saliva and digestive enzymes, and spits it out directly onto your food. Those enzymes start dissolving your lunch where it sits. The bread, the meat, the sugar on a donut chemically melted into a soup the fly can drink. Then it sponges the soup back up. Scientists even have a clean clinical name for the little droplet it leaves behind. A vomit speck.

[00:02:11]And if the fly wants to concentrate its meal, it does something called bubbling.

[00:02:15]It holds a bead of its own stomach fluid on the tip of its mouth to let the water evaporate off, then swallows it back down.

[00:02:21]Vomit, air-dried, and eaten again. And it does not stop to be polite about the other end. A feeding fly defecates almost constantly. It runs food through its body so fast that it eats and excretes at the same time on the same surface.

[00:02:36]Your food. And that is the clean part.

[00:02:39]Now, go back to the feet because this is where it turns strange. A fly tastes the world by standing on it. Its legs are covered in taste sensors, tiny hairs wired straight to nerves that read sugar, salt, and bitterness on contact.

[00:02:52]On its feet, a fly carries at least twice as many taste neurons as it has in its entire mouth. So, the instant it touched down on your lunch, before its mouthparts were even involved, it was already tasting your food through the soles of its feet.

[00:03:05]Feet that moments earlier may have been standing on something you would rather not picture. That is why flies rub their legs together. They are not scheming.

[00:03:12]They are cleaning their tongues. So, the fly leaves behind vomit, waste, and a taste of wherever it has been. But the real payload is invisible.

[00:03:20]In 2017, a team of scientists led by Donald Bryant at Penn State and Stephan Schuster in Singapore did something no one had done before.

[00:03:28]They took 116 individual house flies and blowflies collected from three different continents and ran their entire microbial DNA through deep sequencing.

[00:03:37]Not a swab and a guess. Every genome the fly was carrying read out letter by letter. They published it in the journal Scientific Reports. A single housefly carried up to 351 different species of bacteria. The blowfly, 316.

[00:03:52]A great many of them are harmful to humans.

[00:03:55]And they found where the bacteria ride.

[00:03:57]The legs and the wings held the highest diversity of all. The fly's own body is an airborne shuttle for microbes. And every step it takes, the researchers found, leaves behind a fresh microbial colony, a bacterial footprint pressed into whatever it walks across. Hundreds of steps, hundreds of little colonies on your plate. Among the species they pulled out of these flies was Helicobacter pylori, the bacterium that causes stomach ulcers.

[00:04:23]Add that to everything science already knew, and the common housefly has been tied to more than 100 human pathogens, bacteria, viruses, fungi, and the eggs of parasites.

[00:04:34]It's not the fly.

[00:04:36]It's where it's been. Here is where gross turns into something darker. For a long time, flies spread disease was a suspicion, obvious, but hard to prove.

[00:04:47]Then in 1991, researchers ran an experiment that settled it. Published in The Lancet, they took two Israeli military bases a few kilometers apart, controlled the flies on one, and left the other alone.

[00:04:59]Then they swapped.

[00:05:00]Then they did it again the next summer.

[00:05:03]The results were brutal in their clarity. On the bases where the flies were controlled, fly numbers dropped by 64% and clinic visits for diarrheal disease fell by 42%. Cases of shigellosis, a serious and sometimes deadly gut infection, dropped by 85%.

[00:05:20]Not because the men washed their hands more, not because the food changed, because the flies were gone. Kill the messenger and the message stops arriving.

[00:05:29]Across many studies, controlling flies cuts diarrheal disease by around 40%. In parts of the world without reliable sanitation, the fly on the food is not a nuisance at all. It is a working channel for epidemics, for cholera, for typhoid, for dysentery. And it is getting worse in a way that would have been unthinkable a century ago.

[00:05:48]When researchers in Libya sampled houseflies caught inside a hospital, they found bacteria that no longer respond to our drugs.

[00:05:55]More than half of the Pseudomonas strains on those flies were resistant to at least 10 different antibiotics. And two of the samples were the superbug hospitals fear most, methicillin-resistant staphylococcus aureus, the one you may know as MRSA.

[00:06:10]Flies caught in hospitals in Nigeria were carrying genes for resistance to last-line antibiotics.

[00:06:16]Flies on Indian poultry farms were shuttling drug-resistant E. coli straight out of the animal pens toward people. The fly does not care about the walls we build between a hospital ward and a street. It carries what it lands on, and it lands everywhere. Now, step back because you are outmatched here in a way that is almost hard to believe.

[00:06:33]That fly on your lunch belongs to one of the oldest lineages of animal life still walking the earth.

[00:06:39]True flies, the order Diptera, first appeared around 250 million years ago.

[00:06:44]That is before mammals, before birds, before a single flower had ever bloomed anywhere on the planet.

[00:06:51]When the ancestors of that fly were already in the air, the ancestors of everything you have ever loved were still waiting to exist. And they took over the world. There are around 150,000 known species of fly. Roughly one in every 10 known animal species on Earth is some kind of fly.

[00:07:06]The housefly itself began in Central Asia and followed us to every inhabited landmass in the world. Wherever humans went, it came, too, feeding and breeding in our waste. We even named it for that, filth fly.

[00:07:19]Then there is the arithmetic, which is genuinely disturbing.

[00:07:23]A single pair of houseflies mating in April could, if every one of their descendants survived, produce by August.

[00:07:29]That is a 191 followed by 18 zeros.

[00:07:36]It never happens because nearly all of them die. But it tells you what you are standing against. Right now, there are an estimated 17 billion flies alive for every single human being. You are not winning this. You never were.

[00:07:50]And the individual fly is quicker than you, too. Measurably, provably quicker.

[00:07:55]When a fly watches your hand drop to swat it, it is not seeing what you see.

[00:07:58]A researcher at Cambridge measured how fast a fly's eyes refresh the world, and the number is staggering. A fly can register as many as 250 separate flashes of light every second. You manage about 60. Which means the fast, sharp swing of your arm, the one you are certain it cannot dodge, reaches the fly in slow motion. And in 2008, scientists at Caltech filmed exactly what it does with all that time. In roughly 1/10 of a second after spotting the threat, a fly's brain, a brain the size of a poppy seed, holding about 300,000 neurons, works out where the danger is coming from, plans an escape route, and shifts its legs into the perfect position to jump the other way. It has already decided how it is going to leave before you have finished deciding to hit it.

[00:08:41]So, the next time a fly lifts off your food half a heartbeat before your hand lands, understand what just happened. It watched you in slow motion and left on a plan. Which brings us back to the twist, the one almost every video leaves out.

[00:08:55]After all of that, the vomit, the 351 bacteria, the super bugs, here is what the scientists who study this for a living actually say.

[00:09:05]If a single fly lands briefly on fresh food, and you are a healthy adult, you can almost certainly keep eating. That sounds insane. It is not, because the fly is the messenger, not the disease.

[00:09:17]And one quick touchdown usually leaves behind too little to overwhelm a healthy immune system. The real danger is not the landing. It is the two things that come with it.

[00:09:27]Time, because the longer a fly walks on your food, the more its bacteria multiply into numbers that can genuinely make you sick.

[00:09:36]And place, because a fly that just came off a bin or roadkill or a latrine is carrying a completely different load than one that did not. It's not the fly.

[00:09:45]It's where it's been and how long it stayed, which is exactly why the instinct is right.

[00:09:50]Cameron Webb, a medical entomologist in Sydney, puts it plainly, "Swat it away fast and for most people the risk is small."

[00:09:58]William Kern, an entomologist in Florida, says the worst most healthy adults face in a clean kitchen is a bad couple of days.

[00:10:06]One researcher's private rule is almost funny in how calm it is. Break off the piece where the fly landed and eat the rest. So the honest answer was never throw it out in a panic. It is older and simpler than that. If in doubt, throw it out. If not, eat your lunch. So why does it feel like so much more than that? Why does a fly touching your food set off a reaction wildly out of proportion to the actual risk of full-body recoil, a curl of the lip, a sudden certainty that the whole meal is ruined?

[00:10:35]Because that reaction is not about this fly. It is about every fly and every rotting thing across the entire history of your species.

[00:10:45]In 2011, a scientist named Valerie Curtis at the London School of Hygiene and Tropical Medicine laid out the case in the Philosophical Transactions of the Royal Society.

[00:10:54]Disgust, she argued, is not a mood and it is not manners. It is a system, a behavioral immune system, a second line of defense that lives not in your blood but in your mind, evolved to make you avoid the things most likely to infect you before they can ever get inside you.

[00:11:11]And when researchers lined up what humans find disgusting against what actually spreads disease, the two maps matched almost perfectly.

[00:11:19]The flinch you felt when the fly landed is that system firing.

[00:11:23]It read the fly correctly as a signal of contamination.

[00:11:28]And it yanked you back before your conscious mind had weighed a single fact.

[00:11:32]It has been doing this quietly in every human who has ever lived for far longer than we have had cities or writing or fire.

[00:11:41]We like to treat disgust as a weakness, a squeamishness to grow out of. It is the opposite. That small involuntary no is one of the reasons your species is still here. A creature older than the dinosaurs landed on your lunch and left a trace of everywhere it had been. And something far older than your thoughts read the danger and answered before you could.

[00:12:02]The fly never knew you were there.

[00:12:04]Your body did.