I have an OC, who was born with this one-of-a-kind condition. His immune system is abnormally strong, invincible really.

He's incapable of getting sick, or dying of bacteria, viruses, venoms, poisons, drugs, chemicals, alcohol, radiation, basically any outside substance that can be consumed or injected. This ability gives him some form of semi-immortality.

So my question is, is this possible, or at least close to?

EDIT: to answers some questions, yes he can die from fire or bullets, if they're not related to anything mentioned above, he'll die. I probably should have clarified, just cause he can't die from radiation doesn't mean he can't be affected by it. In my story, rather than dying of radiation sickness like a normal person would've, the mutagens lingered in his system, and slowly began to change him. Finally, he's not really immortal, he still ages normally, he just can't die from pathogens. Also also, forgot to mention his overpowered immune cells also make him heal faster than normal (referring to platelets).

Thanks to everyone for your answers!

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Aug 11, 2022 at 10:41

12 Answers 12


TL;DR: Powers as given: Not Plausible

The Super-immune System

A person's immune system is designed to suss out things like bacteria and viruses, and likely anything that could mimic it enough. It finds an invader, pulls out the biological tools needed to ... handle ... it, disposes of it, and leaves a sort of memory behind should it ever show up again in the form of antibodies.

Second, not sure how much it was thought of, but the human body is home to its own ecosystem of gut bacteria and other things that should be there. Does your super immune system know how to tell the gut flora that should be there from the invading stuff that needs to be removed? If it is a superpower, I presume yes because reasons.

Based on your power's description, it should know to not murder all your OC's gut bacteria. But it may lead to food allergies if the food has a higher than normal bacteria counts (dead or alive) due to processing.

DRUGS! Chemistry in Motion

But chemicals works by deceiving the systems in the body and not actually attacking them. Carbon monoxide binds to the hemoglobin instead of oxygen, and your immune system is none the wiser for it. As a more benevolent example, humble aspirin spoofs bodily functions to prevent the feeling of pain, granting you relief from that sprained ankle. Your immune system is likely not going to catch that either.

Likewise, we can trick various receptors and the like by figuring out the right chemicals that will trick them to do what we want. If you can't make your own neurochemicals, then store-bought is fine.

Likewise a chemical cocktail of a poison/venom might do this or it might just outright destroy things in chemical reactions. While the immune system might nab the dead and destroyed cells as invaders, it will not necessarily tag the stuff doing the destroying. In fact as cells themselves, they may fall victim to the same thing that is doing all the destroying in the first place.


Radiation will do similar -- cause damage without actually tripping the immune system. The only unknown is if the radiation is enough to trigger cancerous growths. Does your immune system know that these irregular and potentially lethal growths are dangerous and Not Normal, so they need to go?

This one is the middle ground -- I can see the super immune system being able to catch it in the early stages and prevent it form spreading. I can also see it not being able to do anything about cancer as it's still the person's body, just not doing what it is supposed to do.

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    $\begingroup$ +1 on middle ground for radiation. I was told during a biochem lecture that each of us then in the room probably had around 20 pre-tumours that would never develop because they'd be picked up by our immune systems. So it's safe to say a normal immune system can recognise some types of cancer at least, though there's ample evidence that it can't catch them all. And the existence of autoimmune diseases might be a clue what happens if you just increase the power to catch more tumours. $\endgroup$ Commented Aug 8, 2022 at 15:35
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    $\begingroup$ The middle ground for radiation is only if radiation-induced cancer is the route to death. Death by radiation poisoning will kill someone regardless of what their immune system can do. $\endgroup$
    – jdunlop
    Commented Aug 9, 2022 at 6:25
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    $\begingroup$ Just want to add one thing: often times drugs work inside the brain, if your brain is changed, they might work less or not at all. As someone who has a changed brain and brainchemistry (meaning being "mentally ill" or neurodivergent) I can tell you that many drugs, who affect the brain, dont work for me. Or only in very high doses. Painkillers, many illegal drugs and even some anesthetics only work for me with very very high doses. Of course drugs affecting the body work normal. Though as a bigger person I tend to need more of those drugs aswell. $\endgroup$
    – bibleblade
    Commented Aug 9, 2022 at 7:41
  • $\begingroup$ The drugs/chemistry in motion aspect reminded me that we'd run into specification issues around things that affect the body in ways that, like coffee, technically count as poisons or venoms, but give us effects we probably want (At least temporarily). So the hypothetical immortal would likely not be a morning person, and almost certainly hates caffeinated coffee, since it won't work for them at all. $\endgroup$ Commented Aug 10, 2022 at 9:52

Not only it won't work as you think, it might actually make them more vulnerable to death by (immune) overreaction.

You see, the inside of our body is a really good place to live in: it is nicely warm and there's plenty of resources such as cells and nutrients for bacteria, viruses and fungi to enjoy to their heart's content. To make sure they don't get the chance to trouble us, our immune systems are here to protect us from outside agents.

The problem? "outside agents" usually only stretches to other organisms that don't belong in our bodies and some types of organic toxins that don't kill us too quickly, and even then there is a large number of exceptions, as our bodies often get in contact with and house many substances and microorganisms which are neutral at worst and helpful at best, such as seen in our gut bacteria, which can offer help in functions that can range from digestion to aiding in our very immunity.

Now, to make sure we're on the same tracks: our immune response essentially works in 3 stages assuming we don't die midway:

  • Stage 0: an external agent enters our body, and it hasn't been detected yet.

  • Stage 1: our immune system detects the thing as a threat, usually by engulfing one of the invaders, and begins an immediate, less specialized reaction (usually causing inflamation) while it takes the information it got by eating some of the invaders to essentially research a more effective response.

  • Stage 2: the immune system cracks the code and crafts antibodies specifically designed to target the invading elements, and so these are produced and released In mass, neutralizing the threat, and the information on it is stored, grating us immunity against it. If the same invader comes back later and hasn't changed enough, the immune system will immediately recognize it and jump directly to the stage 2 response again with little to no side effects or symptoms. This is what we know as a secondary immune response, and it's because of it we'll create to vaccines against diseases, especially those that are usually too debilitating and/or too aggressive for the infected person to develop an immunity in time.

With that said, we come to the main problem: for our immune system to work in the first place, there must be something to identify. And that's not how everything works:

  • ionizing radiation, especially in cases like Gamma radiation, causes damage by essentially breaking you apart on a genetic level. As the thing is a wave and essentially energy, there's nothing to be picked up.

  • heavy metals are a problem and accumulate on our bodies precisely because, despite toxic, they behave very similarly to other non-toxic elements we do need, the difference/problem being that they also work in ways that are detrimental to us. A good example is Carbon monoxide, which forms much stronger links to the hemoglobin than oxygen does, meaning it never lets go, doesn't enter the bloodstream and accidentally leaves that red blood cell unusable for oxygen transportation. Again, there's nothing to detect, and even when there is, it works too similarly to other chemicals to be perceived as a threat, because inherently, it really isn't unless it starts to pile up in quantities considered as too large, at which point it starts interacting with our bodies in ways that mess up too many important chemical processes for our continued existence to be maintained (in some cases "too much" may equate to "virtually anything more than none of it", but still).

Essentially, a realistic super immune system, as far as I understand, would work extremely fast, and could probably make it so you never got anything worse than a minor cold and normally short lasted inflammations, as it'd fight off and develop an immunity faster than the problem could spread, and thus you wouldn't really need to worry about death by viral or bacterial diseases, poisons or venoms, since all of these are natural organic substances your body can and will develop an immunity against under the right conditions. However, it doesn't really help to make you immune to everything, far from it. In fact, it won't even help reducing aging, as the inflammatory reactions caused by our normal immune response is a contributor to that, and the degradation of telomeres, the biggest link to aging we seem to have, isn't really something out immune system can help with. Rather, not getting hurt or stressed is the best way to ensure they don't need to be degraded by "unnecessary" cell division.

There's also one problem you need to consider: an immune system like that can be more trouble than it's worth. Our immune system is pretty crafty, but not the brightest, and like everything in our body, it must act in a balanced way. A weak immune system leaves us vulnerable, but an overagressive and oversensitive immune system can be worse.

He's incapable of getting sick, or dying of bacteria, viruses, venoms, poisons, drugs, chemicals, alcohol, radiation, basically any outside substance that can be consumed or injected.

Is how you described it. But do you know what has a similar description? Allergies. Allergies are basically what happens when your immune system detects and reacts to an outside substance, but in an overagressive and damaging way. Essentially, the immune system is so sensitive that it detects things as threats when they're not, and reacts to them in a violent way, which can even cause someone to die by eating a harmless peanut butter sandwich, or petting a cat. An immune system that targets EVERY outside substance isn't going to make you healthier. In fact, it might kill you depending on how it actually works.

Another big complication is that, as no immune system can be perfect, it can detect harmless things as dangerous invaders, and sometimes, that something is yourself. Auto immune diseases are essentially that: you mistook yourself for an invader and now your body started breaking part of itself apart. problem: you still need that to survive, and when you immune system is violent and efficient enough to grant you immunity to venom in seconds, such a mistake could easily be a fatal one.

So summing it up:

  • while you don't get to age well if your immune system does not work, it's not your immune system that controls your aging. A super immune system doesn't make you age slower, and might actually make you age faster if it's super sensitive as well.

  • if it is or becomes super sensitive though, you might not need to worry about aging, because auto immune responses and anaphylatic shock (an extreme and dangerous allergic reaction to a normally harmless outside substance) might do you in long before your first wrinkles form without proper (and perhaps constant) medical assistance.

  • despite all this, you're still very much in risk of dying from radiation poisoning, mercury poisoning, suffocation by carbon dioxide and other types of poisoning related to heavy metals, as there's really nothing to be detected as a threat, and if your immune system somehow does detect it, chances are that it's already too sensitive for your own good and is already targeting the substances the heavy metals are usually mistaken for.

  • things like fire, deep enough cuts and bullets will also still do you in without medical assistance (and sometimes even despite that). Your immune system is ultimately still organic and carbon based, and it doesn't really work well at over 80 degrees, or when most of your blood is leaving your body.

To get something closer to the kind of semi immortality you want you'd basically need the following:

  • an immune system more like a bat's, being effective but not too keen at causing inflammation.
  • different genetics, including the ability to regrow telomeres in all cells and rapidly divide them in cancer fashion. The ability to edit your RNA like cephalopods do could also help you rapidly adapt to certain things such as temperature changes.
  • extensive genetic repair mechanisms and special protection substances and processes, allowing you to naturally resist certain things, like how targigrades protect themselves against radiation through special proteins shielding their DNA.
  • $\begingroup$ wow, this answere is way to scientific. yeah, no system is 100% safe - we know that since the mid 90's, but: didn't op just want a fishy excuse why his MC would survive like anthrax $\endgroup$
    – user59660
    Commented Aug 9, 2022 at 10:33
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    $\begingroup$ @clockw0rk: If that's what they wanted, they should have specified limits on their required "immunity". They specifically included all "chemicals" in general, although they did limit it to inject or ingest, so they probably weren't considering gases like carbon monoxide. But it does include simple chemicals like cyanide compounds which could be ingested or injected, which prevents the body's cells from using oxygen. That's not an immune-system thing at all. Neither is radiation in high enough doses to kill you outright, not just cause cancer. $\endgroup$ Commented Aug 10, 2022 at 4:13

Moving the Goalposts

Since the general consensus is "not possible" maybe just move the goalposts a bit and think of how it (or something effectively similar) could be made to happen (since this is Worldbuilding and not Biology). This would depend very much on your setting.

Victorian steampunk: the character is just really really good with medicines, antivenoms, etc. I've seen stories that have assassin characters that are nearly unkillable simply because they always have a counter for everything. Most poisons and diseases don't kill them because they know how to counteract it and have what they need on hand to do it. They would protect themselves from carbon monoxide poisoning because somewhere on their person is a crude detector that warns them if CO levels are too high. Etc.

Futuristic sci-fi: nanobots. They have medical nanobots that take care of everything. Maybe it's alien tech. Maybe they're the developer of the tech. Maybe they're an experimental super-soldier. However they acquired it, the medical nanobots can't necessarily sew an arm back on but they can handle nearly any infection or poison.

Magic: because wizards.

For modern day you could potentially go with the steampunk answer or the nanobot idea since both are at least vaguely plausible (we could probably do nanobots today if we hand-wavium a tiny battery technology that holds 10,000% more than current technology. Quantum tunneling something something. The mechanical design is within our ability, we just lack a power source.)

Or, you know, you go the Wolverine route, but that's effectively magic.


Not possible at all

An immune system simply can't protect against everything. A peime example is HIV. This attacks the immune system to great effect, using it to spread further. But rabies is also a good example. They infect neurons, which have a way to signal an attacking immune cell to destroy itself. Or certain amoeba, which can be so large they simply attack the killer cells by sheer force.

Poisons and the like are largely out of the jurisdiction of the immune system. They are chemicals that do things in our body. This is good, as by eating we can use these chemical processes to do anything at all. Some chemicals disrupt this in any number of ways. They bind to neurons preventing messages or stimulating too much. They simply disolve or destroy cells. They interrupt mitosis or the mitochondria. It can do a lot. There is no guarding against it, as many poisons just use the system we absolutely need. They interact and just happen yo have unintended side effects.

What if it was possible?

Let's just say that your immune system does do what you say. It prevents all. Then we're not out of the woods yet by a long shot.

Ever wondered why our immune system isn't fully functioning at all times? You might understand that increasing the heat is bad for most of the bidy, but why not leave an army of immune cells active to prevent any threat?

The problem is that the immune system is actually damaging and draining on the body itself. The immune system isn't like a good army that protects it's citizens. It is more like wild carpet bombing your own cities to destroy the infiltrated enemies. It isn't healthy at all. That is why our immune system is mostly at a low ebb, enough to deter a large amount of invaders, but not enough to stop all.

Now imagine you put the immune system in just a bit higher than normal activity permanently. It can go ok for years. Decades even. Then suddenly there's burn out. Your immune system has depleted you, causing a major problem for your body.

Do not try to make the immune system an all powerful super whatever. This is impossible. All in all our immune system is already incredibly powerful. It is also dangerous.


It is not possible to be immune to all poisons as there are so many of them and many toxic chemicals cause strong redox reactions that won't give a damn about biochemical niceties. For example Chlorine Trifluoride is one of the most reactive substances known, it will set fire to concrete and is spontaneously flammable with water. It is very nasty stuff and would burn down any biochemical immune system to fluoride ash.

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    $\begingroup$ Don't forget dioxygen difluoride, more amusingly referred to as FOOF. Because there isn't an obligatory XKCD directly related, we'll fall back on a What If that contemplates mixing H₂S with FOOF, which would produce tissue-dissolving hydrogen fluoride gas. $\endgroup$
    – Doktor J
    Commented Aug 8, 2022 at 19:48
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    $\begingroup$ Yes FOOF is nasty indeed, although its not stable at room temperature. Chlorine trifluoride is stable at room temperature and rather foolishly perhaps experiments have been carried out to see if it could be used as a rocket propellant (see the book Ignition! by John D Clark) on page 67 he describes what happen when a 1 tonne steel cylinder of CTF that was being cooled in dry ice had become embrittled and cracked dumping the CTF on the floor. "it chewed its way through 12 inches of concrete and three feet of gravel underneath, filled the place with fumes that corroded everything in sight" $\endgroup$
    – Slarty
    Commented Aug 9, 2022 at 10:15
  • $\begingroup$ Indeed, for certain poisons which act mechanically and/or chemically, there is nothing an immune system can do against. When things stop being biology and start becoming physics, there is nothing your cells can do about it. $\endgroup$
    – vsz
    Commented Aug 10, 2022 at 6:50
  • $\begingroup$ @Slarty I don't mind that it's not room-temperature stable. I mind that it can make liquid methane explode - sorry, "react vigorously" - at 90°K. And the less said about how it reacts with hydrogen sulfide the better. Let's just say that there are easier, less catastrophically destructive methods for synthesizing hydrogen fluoride. $\endgroup$
    – Corey
    Commented Aug 10, 2022 at 23:21

Not plausible.

The term poison ranges from complex nerve agents to something as simple as carbon monoxide binding to hemoglobin. An improvided immune system is something different.

The effects of improved immune protection might somewhat plausibly be lumped with an increased resistance to mutagens by improving self-repair mechanisms. Having the same explanation fight both bacteria and simple chemical compounds in the bloodstream is less probable.



I can't see how your OC could be possibly be immune to everything, because there's one specific type of infectious agent that cannot be fought with anything that isn't 100°C+ temperatures and strong bleach: a prion. Prions aren't killable microorganisms. Rather, they're misfolded proteins that make other proteins misfold - think of them as zombies that are even smaller than your body's cells.

I strongly doubt your hypothetical person's biology, however unique it is, can fight off an infectious agent that turns the very building blocks of their cells into more infectious agent, and see absolutely no way by which this could realistically happen within the realm of conventional biology.

After all, it is rather difficult to kill that which was never alive in the first place...

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    $\begingroup$ just to be clear the immune system can combat prions, it is just very ineffective against nervous system prions, which is not surprising, its fairly bad at fighting any disease that reaches central the nervous system. $\endgroup$
    – John
    Commented Aug 8, 2022 at 12:20


Your hero ain't from around here. He was raised as a human and his body was meant to pass as human under cursory exam but he is not human and not even a biological organism. Germs, toxins and drugs don't work because the inner functions of his body are not at all like any terrestrial life form and may not even be carbon based.

Or at least outside influences don't work as one would expect them to work. The thing that he is might be still be affected by certain outside influences. Those things and the effects that they have on your hero will probably come as a complete surprise to all involved, at least the first time.

  • $\begingroup$ Since this worldbuilding...how many works exist out there that are just like this? Highlander comes to mind. $\endgroup$
    – rtaft
    Commented Aug 10, 2022 at 19:27
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    $\begingroup$ @rtaft - I was thinking Superman. $\endgroup$
    – Willk
    Commented Aug 10, 2022 at 19:50

Your hero needs a hazmat suit or some equivalent

If you want to avoid poison or pathogens, don't take in things from the environment, including air, water, or food. Also, you'll probably want to cover your skin with something less permeable. Probably don't have sex either.

It is effectively impossible for a living thing to be immune to all poison and pathogens. A better strategy is to keep the poison or pathogens from getting in the first place.

Living things are complex machines that are constantly trying to maintain their insides under the right conditions for life: the right temperature, the right amount of water, salt, glucose and other chemicals. Living things contain thousands of different proteins and other molecules that do thousands of different jobs to keep a living thing alive. Hence, there are thousands of different ways to break a living thing, which are called mechanisms of toxicity. Drugs and poisons work by interfering with various processes, often by binding to a protein. As mentioned in another answer, carbon monoxide kills you by binding to the iron ion in the heme of hemoglobin (a protein), which does not allow oxygen to bind and you essentially suffocate you from inside. Nerve agents kill you by binding to the protein acetylcholinesterase. Under normal conditions the job of acetylcholinesterase is to remove the neurotransmitter acetylcholine from nerve synapses, allowing nerves to turn off after being excited. If you cannot clear the acetylcholine, the nerves cannot turn off, so muscles cannot stop contracting.

Bacteria, viruses, and cancer kill by replicating in ways that disrupt important biological processes (or sometimes due to overreaction of the immune system). This is the problem with your superimmune system. The immune system is a balance between recognizing and destroying pathogens and being overzealous and destroying your body (see autoimmune diseases).

So the point is that there are many chemicals that can kill you. In fact most chemicals can kill you. Too much salt. Too much water. However, for poisons (or pathogens) to kill, they must enter the body. The most dangerous poisons are those that enter the body through the air (carbon monoxide, hydrogen cyanide, nerve agents). Also, some of the most dangerous and contagious diseases are passed through the air (influenza, COVID-19). A good way to avoid these poisons and diseases would be by not taking in air from the environment. However, living things use oxygen and sugars as a source of energy. So your hero would either need to carry their own safe oxygen supply or have an air purifier/sterilizer. So basically they would need a hazmat suit. It even possible that this hazmat-like equipment could be biological, although it might take more energy to run than required by your usual 2000-calorie per day human.

Taking in oxygen could be avoided if your hero had a different source of energy, but this would require radically non-human biology. One option that exists on Earth already is a sulfur metabolism. Maybe your hero could carry around a block of sulfur. Other more exotic options are obtaining energy from a pellet of plutonium or some kind of nuclear energy source, so your hero would not need to breath air or eat to obtain energy (which would avoid another inlet for poisons and pathogens). This would require some radically different biology from life on Earth.


Lots of answers already. Yet, I must give a dissenting opinion.

Diseases: YES

Robots are immune to all diseases. Computer viruses don't count. It's possible (not plausible) to have an alternate biology that would be immune to all Earth's diseases.

Furthermore, any one-of-a-kind biology can potentially be immune to all infectious diseases, simply by the virtue of being one-of-a-kind. Cross-species diseases are much less common than specialized ones.

Aliens might be wiped out by simple microbes like in War of the Worlds... or they might be totally immune. We don't know, it really depends.

The biology of any creature immune to human diseases would have to be incompatible with humans to a great degree. For instance, they would probably be unable to consume many human foods. There is zero chance of interbreeding.

It's definitely not plausible for someone to be born this way to normal human parents - their mother wouldn't be able to carry them to term. Also, changes like that don't just happen randomly. You're really inviting intelligent design here.

Poisons: NO

Any biological life is only possible through chemical reactions. It takes A LOT of reactions, too. And they have to be within a specific band of equilibrium.

Anything that can react, can overreact. Any food that can sustain life, can be burned - it might not burn easily, but it will, that's how they measure food calories.

Life involves the interaction of open systems. Any living organism must receive building materials and energy for its growth and renewal in some way. That means it must have a way for chemicals to enter, to react with other chemicals inside it. Even a hazmat suit has to be taken off eventually.

The only way to be invulnerable to all poisons is to be dead.


Diseases: Yes

I would argue that anything that your body resists via its immune system could potentially be harmless, if your immune system were fast enough. The human immune system is already capable of responding to more pathogens than even exist on earth because of the variability of segments that make up antibodies. The main reason we get sick is when the infection-causing agent gets enough of a foothold that it exhibits symptoms before your body can get rid of it. You touch, inhale, and ingest possible pathogens every single day, but you don't get sick because usually your body's defenses destroy them before they can multiply enough to feel the effects. If your immune system were fast enough at cranking out antibodies and the requisite immune response cells (and those cells were effective enough at disabling/destroying the pathogen), you could conceivably survive any pathogen at all, even ones that no human has ever encountered before.

Chemicals: Maybe?

Chemicals, poisons, or toxins are a bit harder to explain away, because elements/chemicals are fundamental to the functioning of your body. In fact, the reason that many chemicals kill is because they disrupt existing processes by masquerading as other important elements, usually through similar shape or electron configuration. For example, lead takes the place of calcium in proteins and other cellular products but ultimately doesn't function the same way as calcium (misfolded proteins, etc) and can't be easily flushed out by normal cellular processes. Carbon monoxide does the same to oxygen, especially in your hemoglobin where it binds more tightly than oxygen, eventually starving all your cells of the oxygen they need. You can't just be immune to lead or carbon monoxide the way you can to a virus, because antibodies aren't involved, and you can't just make your whole body ignore something that molecularly appears to be the same as something it needs. Some chemicals only cause harm while they are circulating through your system, but are able to be broken down by your liver or excreted by your kidneys. Having super fast liver enzymes (like your fast immune system) or hyper-efficient kidneys could cover much of the problems of poisoning. Radiation kind of falls into this category. Since the biggest problem with radiation is the damage it causes to your cells, having super fast/efficient cellular machinery could mean that you can repair the damage before it has a chance to accumulate and cause symptoms.

Ultimately, this is a work of fiction, so how much explanation you want to give is entirely up to you. Maybe your character has studied the processes enough to know their immune system is just really fast and strong, but have no idea how they resist chemicals or radiation. You can leave this as a plot hole, or lampshade it by talking about the theories they had about it and why those aren't true and that they haven't found the full cause yet. Some would argue that hyper fast cellular processes means you could be actually more susceptible to some poisons, so covering that is up to you. The other thing to realize is that they don't have to be 100% resistant to every possible virus, bacteria, poison or toxin. They only have to have resisted the ones they encountered. That also leaves the opening for the antagonist to find the specific right means for taking down your hero, knowing their supposed invulnerability.


Not possible, but ...

You can approach this state by making your character an alien.

His biochemistry is subtly different to ours in just about every respect. This isn't something that could happen by random mutation of a human genome. Detailed analysis would soon reveal that he isn't any sort of Earthlife at all.

He can eat the same food, because he doesn't need any essential amino acids or vitamins that aren't available here. But our bacteria are woefully ill-prepared for countering an immune system utterly unlike anything that ever existed on Earth, so they get nowhere(*). And of course his genetic code is different so viruses can't replicate in his body at all.

As for poisons: yes, he can be poisoned. But the chemicals to which he is sensitive are by and large different to the ones that we are vulnerable to, so poisoners get disappointed. Maybe nerve gases have no effect whatsoever on him. (They affect particular enzymes shared by most multicellular Earth life, which he doesn't have). On the other hand, he won't fare much better than we would if he fell into a large vat of acid. And it may be that a dose of, say, copper sulphate, that would give us the runs at worst, might be a lethal danger to his copper-intolerant metabolism. But those who wish him harm, don't know this. (Not so very far-fetched: Earth fungi and moulds are intolerant of copper, which is why copper carbonate a.k.a. Bordeaux mixture has been used as a fungicide).

If you are thinking Superman ... yes, the fictional details are risible but the concept is closer to the mark than many realize!

(*) This is the optimist's scenario. The pessimist's scenario, is that microbes have the upper hand and only immune systems co-evolved with them can keep them at bay. So if we ever meet aliens, the result is deadly: their microbes reduce us to stinking goo, while our microbes do the same to them. HG Wells War of the Worlds was an intermediate scenario, where Earthly microbes and immune systems were triumphant over Martian ones.


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