T

he world’s deadliest animal doesn’t have fangs or bone-crushing coils. It doesn’t even have teeth. But it can inject parasites and viruses straight into your bloodstream.

What is this killing machine? None other than the humble mosquito.

That, at least, is what Bill Gates wrote when he put out a list of the world’s most deadly creatures.

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The list sparked controversy because, technically, it’s not the mosquito — or the freshwater snail, or the tsetse fly, or the dog — that kills. It’s the viruses and parasites these critters transmit. Judge the human species by the same standard and we would be far and away the most lethal creature on the planet, because we transmit deadly diseases such as HIV and tuberculosis to one another, as Jonathan Eisen, a biologist at the University of California, Davis, noted in a blog post.

Point well taken.

Another important point: Most of these killers disproportionately affect poor people in the developing world. Not only do they often live in close contact with flies, mosquitoes, and water-dwelling worm larvae; it’s also harder for them to get proper medical care.

All that said, we at STAT were curious about how, exactly, the creatures on Gates’s list do us in. So, taking Gates’s numbers at face value, here’s the science behind the scary statistics.

Mosquitoes

Death toll: 725,000 people a year

When a mosquito sticks its proboscis into your skin, the tip wiggles around, looking for a blood vessel. Once it hits a jackpot, the bug injects its saliva to prevent the blood from clotting. But the saliva can contain stowaways — and they’re the ones actually doing the killing. Most often, deaths occur because of Plasmodiumthe genus of parasites that cause malaria, which reproduces first in your liver and then on your red blood cells. Deadly viruses like dengue, chikungunya, and yellow fever are also masters at hitching rides with mosquitoes.

Humans

Death toll: 475,000 people a year

Turns out we’re perfectly good at doing other humans in, even without help from lions. Or mosquitoes. Most evolutionary psychologists see our murderous leanings as a natural outgrowth of the aggression that helped our ancestors stay alive and protect their offspring, even if killing wasn’t the ultimate goal. But a few researchers argue that natural selection perpetuated or even honed our capacity to kill, since it may have been evolutionarily beneficial to murder in specific situations.

Snakes

Death toll: 50,000 people a year

Doctors Without Borders calls the fallout from snakebites “one of the world’s most neglected public health emergencies.” The incidents often take place in rural areas, making it hard to reach medical care before the venom causes irrevocable damage by attacking your central nervous system, shriveling your tissues, or causing you to bleed profusely. The wide variety of snakes, and symptoms, makes it hard to develop an effective treatment. Still, one enzyme called sPLA2 may be prove an effective drug target — and there’s already a biotech company trying it out.

Dogs

Death toll: 25,000 people a year

Man’s best friend is also one of our worst enemies. Your pooch’s saliva can contain the virus responsible for rabies; if it does, the virus will travel up the dog’s nerves and into its brain, making it unusually aggressive and more likely to bite. Oh, and it will also become terrified of water and unable to swallow properly. Undiluted saliva means more virus present in every last drop. Just when you thought it was safe to pet your poodle …

Tsetse flies

Death toll: 10,000 people a year

This large brown fly often has an unseen passenger in its gut and its saliva: an immature single-cell parasite called Trypanosoma brucei. A bite from the fly will inject this parasite into your body. As soon as it’s in, the single cell lengthens out — picture a leech becoming long and graceful to swim — and can burrow into your tissues and your central nervous system, causing sleeping sickness.

Assassin bugs

Death toll: 10,000 people a year

This is a particularly nasty one. The assassin bug sits on your face, drinking your blood and it has no qualms about defecating at the dinner table. The bite itches, so you scratch it and in doing so, you brush those tiny feces into your mouth, eyes, or the broken skin from the bug bite. That’s bad news because the feces contains parasite called Trypanosoma cruzi, which can wreak havoc by causing Chagas disease, sometimes causing your heart or intestines to swell up. Assassin bugs are widespread in Central and South America, and are present in the southern United States, but tend to afflict mostly those who can’t afford screens and secure roofing to keep out unwanted pests.

Freshwater snails

Death toll: 10,000 people a year

Snails might seem pretty harmless. But they harbor a particular type of worms that are released into bodies of water as larvae. Step into the water to bathe or wash your clothes, and those larvae can penetrate your skin by degrading its proteins. They won’t leave a wound. But they will leave you with a terrible illness called bilharzia or schistosomiasis.

Correction: An earlier version of this story misstated the dates for the Gates and Eisen blog posts.

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