Smallest living thing that can kill a human?

Earlier today while watching a quite non-deadly wasp on my patio, I was wondering what the smallest organism capable of killing an average adult human might be. To keep things in the realm of what I’m thinking, some ground rules:

  • Let’s leave out anything that has to reproduce to kill an adult human, so a single Ebola virus or bacterium of some sort is out.
  • Allergic reactions don’t count, unless enough humans have the allergy that it can be included in the “average adult human”.
  • The entire organism counts, so if some thorn produced by a tree is deadly (as an example off the top of my head), that wouldn’t count.

Any ideas?

Well while I wa in the Boy Scouts they told us that the Brown Recluse spider was pretty damn small and deadly. Had me scared of those little buggers for some time.

The brown recluse spider is pretty small and it can kill an adult human in rare cases.


Red back spider?

Funnel web spider?

Fire ant?

Funnel webs are relatively large compared to some other deadly critters, and I don’t think a single fire ant can kill an average adult human (I doubt a single fire ant can kill an average infant human, actually; their damage comes from their numbers). I don’t enough about the other two to say, though.

My guess when I tried to come up with an answer myself was the box jellyfish, but I haven’t found details on its size yet.

I guess technically we need at least two categories to define “smallest”:

  • Lightest (smallest weight), and
  • Smallest in volume

While the box jellyfish might not qualify for the second, I wouldn’t be surprised if it turns out to be the lightest.

The box jellyfish can have tentacles up to 80cm in length. Perhaps a better bet would be the Irukandji, another deadly jellyfish which is only 2.5 cm in diameter.

Yup, the Irukandji was what I was thinking of. I’m not sure whether I got it confused, or Discovery/Animal Planet/National Geographic/whatever I saw it on most recently mixed the two up. I remember seeing something that looked far more like the Irukandji link than the box jellyfish link.

And after reading more in your articles… the red back spider is only 1 cm long (the deadly females, that is; the non-deadly males are even smaller)! Can anyone come up with anything smaller? That may be the volume winner, but, again, I wouldn’t be at all surprised to find the jellyfish contender is lighter.

At 2 centimeters long the Strawberry Poison Dart Frog is considered to be among the most poisonous animal on the planet (I think the Golden Poison Frog is deadlier but it is bigger at ~2").

That said they may not count because they are not poisonous by themselves. That is…they do not produce their own toxins but seem to get it from their diet. As such poison dart frogs can be raised in captivity and fed a different diet than they get in the wild and not be poisonous at all. However, this little guy in the wild can be positively lethal.

Are we going for potentially fatal or certainly fatal? Red Back Spider bites are not necessarily fatal to a human. Serious to be certain and death is a real concern but you may well live…no one has died from a Red Back bite since 1956 since antivenom was available (barring an allergic reaction).

Near as I can tell the poison dart frogs are definitely lethal to a human. On the flip side the frogs do not attack people so a person would have to touch one and then put their hands in their mouth (or otherwise ingest the toxin…there are reports of people dying from drinking water where a poison dart frog have been) or have an open wound where they touched the frog to get the toxin in their system. If they do however they are in big trouble.

The problem, as with most of these questions, is that it’s way to vague to allow any definitive answer.

For example if a person were immobilised and a mouse chewed through the exposed artery in the ankle they would die within minutes. In reality that’s about as likely as a chocolate kettle, but its possible. At the next level a single fire ant could indisputably kill a healthy adult… if it was swallowed and stung the victim on the right part of the throat on the way down. Airway swells shut, victim dies. But does that count? It’s not a highly likely scenario but it’s entirely possible and I wouldn’t be surprised to find out that someone has dies in exactly that manner. After all plenty of people have been killed or come near death from swallowing wasps in that manner.

And the possible scenarios form a continuous spectrum. Someone has already mentioned death from frog poisoning. And an Irukandji could kill a person, but it’s along way from being a certainty. It requires a specific set of circumstances, not least of which is that in reality you need a lack of care so the person drowns.

Which brings me to the next question: what care or negligence is allowable? Do we assume basic first aid or and ICU or a naked person tied down and immobilised with drugs? That will affect the possible answers significantly.

My personal guess would be Clostridium botulinum. It produces and incredibly potent and stable toxin, and it does so indefinitely. If you could isolate a single cell engineered to be non productive it would, given sufficient time, produce enough toxin to kill a person. A single prokaryotic cell can kill most healthy adults under those conditions. But once again we have the problem that the actual scenario isn’t going to occur under normal circumstances.

I could give any number of other strong contenders but they all depend in the exact ground rules.

A mosquito carrying Malaria, West Nile Virus, etc. seems like a safe bet.

But in that case it’s the virus that does the killing, and the OP ruled them out.

I beleive a Black Widow can kill the young and elderly. They are quite small.

I’m thinking along the lines of something that can bite or sting me once on, say, the hand, and the venom (or whatever, as long as it isn’t alive) it injects, without reproducing, would kill me if I didn’t receive some sort of anti-venom. There are contrived circumstances that make things other than what I’m looking for win (here’s another: a mosquito is somehow loaded up with prion-filled blood, and injects that blood into me; the prion isn’t a living thing, so the mosquito wins assuming I die years later), but I’m looking for something along the lines of that first sentence. I guess another stipulation is that death has to be rapid enough or associated with enough pain from the bite/sting that I can reasonably know what did it (as opposed to a sting that introduces something that kills me years later).

So the fire ant is still out, but the irukandji is still in the running, at least by the information in that link… unless the victim mentioned died “a few days later” from drowning, but I can’t see how that’s possible.

I’m not sure whether red-back spiders beat out black widows; one source gives the size of a female (deadly) red-back at “about 1 cm,” and Wikipedia has the black widow at “about 0.5 inches (13 mm)”. That size goes up to “1.5 inches (38 mm)” when you count the legspan, and I’m not certain if the legs were counted for the red-back.

Anyhoo, carry on. Thanks for the suggestions so far. The red-back is winning as far as I can tell, but I’m not certain of that just yet.

Well, a virus made out of antimatter could kill you. And probably most of your neighborhood. I’m just trying to cover all the bases here…

What about ticks? I hear about 2 people in Australia die every year due to tick bites.

The toxin released by a single botulism microorganism can kill you.

I win. :smiley:

I don’t think so. An HIV virus* is about 60nm in radius, so assuming a density of 1, its mass is 5x10[sup]-19[/sup] kg. Annihilation of this much antimatter releases 0.1 Joules of energy. That’s 0.02 calories, i.e. enough to raise the temperature of 1/4 teaspoon of water by 0.02 degrees. (Unless I did something wrong.)

*Yes, I know that’s redundant. What am I supposed to say, “an HI Virus”?

How about a blue ring octopus? It’s very, very poisonous. It carries enough of that poison to kill 26 adults within minutes. And its *maximum * size, fully spread out, is about 200mm.


Onset of nausea.
Hazy Vision. ( Within seconds you are blind.)
Loss of sense of touch, speech and the ability to swallow.
Within 3 minutes, paralysis sets in and your body goes into respiratory arrest.
The poison is not injected but is contained in the octopus’s saliva, which comes from two glands each as big as its brain. Poison from the one is used on its main prey, crabs, and is relatively harmless to humans. Poison from the other gland serves as defense against predators. The blue-ringed octopus either secretes the poison in the vicinity of its prey, waits until it is immobile and then devours it, or it jumps out and envelops the prey in its 8 tentacles and bites it.