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If the immune system of a healthy human just shut down completely and instantly (magic, nanotechnology, whatever), and after a certain time period it went back to normal, how much time without the immune system would be survivable?

There are real-world examples of people with severely damaged immune systems, but I don't know how total it was.

So, basically:

  1. What length of time would be immediately or almost immediately fatal? (so even restoring it completely would not save from a quick death)

  2. What length of time would be survivable but lead to long term (or permanent) consequences?

  3. What length of time would be completely healable afterward, even without modern medical treatment?

  4. What length of time would be unnoticed or almost unnoticed?

Assume the victim of this curse is not doing anything dangerous or unsanitary during its effects. (so no cuts, bruises, being in an unsanitary environment, etc.)

Of course, as we have no real-world data to measure it (AIDS-patients don't go from 100% to 0% in an instant), a rough Fermi estimation would be enough, but even for that, I have no idea where and how to start.

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    $\begingroup$ Without stating the environment or whether the person so afflicted is going to get any medical treatment, it's going to be difficult to say. Medieval Europe is a much different place to modern Europe, and even the life experience of the victim will make a big difference. $\endgroup$ – Monty Wild Sep 29 at 18:15
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    $\begingroup$ Human immune system is a complex thing. What exactly do you count as "immune system", for the purposes of this question? I suppose you don't consider for example stomach acid, or all the micro-organisms living in our skin or intestines as part of the immune system here, or do you? For reference: en.wikipedia.org/wiki/Immune_system $\endgroup$ – hyde Sep 30 at 4:57
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    $\begingroup$ Are you assuming all the bacteria etc. that live on and inside the human also shut down instantly? If not, there's probably going to be a relatively quick affair; if yes, restoring the immune system by magic again will not be able to revive those, which will have serious implications on its own, regardless of the status of the "immune system". $\endgroup$ – Luaan Sep 30 at 10:13
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    $\begingroup$ define "completely", having skin is a large part of your immune system, it acts as a barrier, are we assuming a person without skin? $\endgroup$ – John Sep 30 at 13:38
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    $\begingroup$ It should be noted that if your immune system were abruptly shut off for 12 hours and then just as suddenly turned back on you would likely die of shock as the immune system suddenly "attacked" all over your body. $\endgroup$ – Hot Licks Sep 30 at 18:36
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Within 24-48 hours you'd start feeling the effects of skin conditions from unchecked staphylococci and the like, such as eczema and erythrodermatitis.

It is difficult to come up with a hard, reliable source for infection progression since instantaneous immune collapse isn't really a thing. However, ex novo infection can have an incubation as short as 96 hours and in-vitro growth time is reported as about one day (google 'staphylococcus growth time' or 'incubation period').

As soon as the skin barrier is broken, cellulitis and circulation problems will ensue, followed (since there's nothing to check the infection) by gangrene. Within three days you will be severely disabled and within four or five days pretty much be unable to move. Expect death from septic shock to occur within one week. One week, in this case, refers to a time period not exceeding 7 Earth-days.

Time-frame data is also difficult to obtain. I have used data from several instances of severe Clostridium infections, where "symptoms usually develop six to 48 hours after the initial infection and progress very quickly" in compromised subjects.

So:

  • 0 - 24 hours: No real effects except itching and rashes
  • 24 - 48 hours: Weeping sores, discoloration and loss of sensation in limbs and other areas. Some loss of functionality may be noticeable after healing.
  • 48 - 72 hours: Severe pain and gangrene. Even after immune system recovery, there are strong chances of amputation being required, scarring and permanent loss of some functionality.
  • 72 - 96 hours: Almost certain amputation necessary, possible death by septic shock even after healing.

So, while in that time several nasty kinds of cancer are guaranteed to have sprung up, they won't be what kills you.

(If several brief immune loss periods repeat, however, it's possible for some of those cancers to develop enough to become dangerous).

Meanwhile, however, dormant infections might spring up and manifest (herpes, for example, and some fungal conditions). These will take longer to disappear even after healing (actually might surface some time after the immune system has recovered).

If you have a preexisting condition or eat anything that requires immune support (the bactericidal properties of saliva will still be there, but several kinds of spores will not be killed and usually die when they germinate and get recognized by the immune system - that barrier will no longer hold), then anything from cholera to Montezuma's Revenge can kill you within 48 hours.

If you don't have complete immune deficiency, then you can survive indefinitely as long as you take sufficient precautions (this is the so-called "Bubble Condition" or "Bubble Boy Disease").

Make it more severe than that and you have something not too different from Acquired Immunodeficiency Syndrome - AIDS.

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    $\begingroup$ While I find your answer to be totally plausible, it would be helpful to point to some kind of source. What your answer made me realize, is that as meat (as in food) starts to spoil after a day or two if not refrigerated (and, depending on the circumstances, even sooner), our bodies would react in a similar way. Was your logic based on that too? $\endgroup$ – vsz Sep 30 at 4:06
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    $\begingroup$ @vsz no (I added pointers to my actual "research"), and actually your observation is sound and makes me fear I've been too optimistic in my timeline. On the one hand, food meat has no skin barrier, which I expect to add around two or even three days' worth of endurance time. On the other hand, food meat has no working circulatory system to help spread bacteria, and is not at a comfy, incubation-friendly 37 °C. My estimates could well be off. However, instantaneous immune collapse isn't a thing, so I feel those estimates are defensible in principle. $\endgroup$ – LSerni Sep 30 at 6:27
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    $\begingroup$ @NathanLong I'd assume that breathing and eating also helps (by keeping the cells alive and functioning). $\endgroup$ – Dan M. Sep 30 at 14:23
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    $\begingroup$ first 24 hours-itching and rash: Is itching/rash not an immune response? $\endgroup$ – Mr.Mindor Sep 30 at 15:21
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    $\begingroup$ @Mr.Mindor only partially, in both cases (normally). But now that I think of it, it becomes difficult to say what is an "immune response" and what isn't - rash for example is increased blood flow, a response to both an immune-mediated histamine signal, and a cellular-level reaction by attacked cells. Is this latter an immune response? In a sense. I'd like to say "yes"... but, if I do, then "no immune reaction" means that the cells die quietly and without influencing the nearby cells, which seems preposterous. $\endgroup$ – LSerni Sep 30 at 16:13
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Since your body contains at least as many microorganisms of various kinds as it does human cells, which are continually kept in check by the immune system, I’d say that “a few hours” was a reasonable guess for your survival time with no immune system at all. You wouldn’t have to wait to be invaded by bacteria or viruses, the ones that would kill you are already there.

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    $\begingroup$ Even in medieval Europe, where they had no germ theory, you'd have a couple of days. However, the germs already present on your body are not necessarily well placed to take advantage of an immune deficiency. The knowledge or otherwise and habits of cleanliness of the afflicted person will affect how quickly they gain access. $\endgroup$ – Monty Wild Sep 29 at 18:20
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    $\begingroup$ @MontyWild they didn't have germ theory but they did have immune systems. What OP asks is, I believe, something a bit different. $\endgroup$ – Gnudiff Sep 29 at 18:52
  • $\begingroup$ Depending on the reading of the question, those micro-organisms are part of the immune system, and might get shut down too. Or not, depending on interpretation. $\endgroup$ – hyde Sep 30 at 5:01
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    $\begingroup$ "A reasonable guess" isn't what I would call a science-based answer. $\endgroup$ – infinitezero Sep 30 at 15:44
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    $\begingroup$ @infinitezero Since there’s no such thing as a human being whose immune system has been completely and instantly turned off, guesses are the only option. There’s no science on the actual subject, just some indicative science, which I quoted. $\endgroup$ – Mike Scott Sep 30 at 15:53
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There have been cases such as that of David Vetter who’s adaptive immune system was non-functional from birth. Due to the death of a previous sibling, the parents knew that there was a 50/50 chance of the same condition affecting any further children they had, but they decided to have another child anyway.

That child was David Vetter who unfortunately was affected. As the risk was already established he was born in a sterile environment and transferred to a sterile bubble within 20 seconds of birth.

He lived for 12 years inside the bubble but at the age of 12, he was able to receive a bone marrow transplant from his sister. Unfortunately due to complications, he had to be removed from his plastic bubble and he died 12 days later due to a viral infection.

https://www.rarediseasereview.org/publications/2017/8/7/living-without-an-immune-system

The key questions are what environment is that person living in and what aid might they receive to help them? Perhaps the above might be seen as a “best” case, had David been born in a pigsty without medical intervention of any sort I suspect his survival time would have been measured in hours. To answer your specific questions:

  1. Assuming no medical intervention or specialist care death would probably follow within days.

  2. Permeant consequences might or might not result from an infection at any point depending on the nature of the infection. But assuming infection by agents that would normally be easily defeated by the immune system, permeant consequences would probably only arrive in the latter half of the infection period before death and not with certainty even then.

  3. The length of time required to heal when the normal immune system returned would also depend on the nature of any infection and especially the amount of time it was left to multiply before the immune system returned. Assuming an infection by easily defeated agents, this recovery period might range between the equivalent of having a cold or flu to pneumonia or septicemia.

  4. The amount of time that the person might not notice a missing immune system would also depend on the environment and the nature of any infection. An exact answer again is not possible, but many minutes could easily pass in a relatively clean environment, but probably not many hours.

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    $\begingroup$ David Vetter seems to be solid evidence that people worried about their natural microbiome are wrong. $\endgroup$ – Ryan_L Sep 30 at 15:36
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    $\begingroup$ @Ryan_L, no, it isn't. David Vetter didn't have a microbiome. Normal people do, and when it gets out of balance, bad things can happen -- just not the sort of bad thing most people who are worried about their microbiome are worried about. See, for example, Clostridioides difficile, which is normally kept in check by other gut bacteria. $\endgroup$ – Mark Sep 30 at 20:37
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Your lungs, warm and moist as they are, are a fantastic growth medium for any fungal spores & bacteria you might inhale.

It simply isn't a problem for us with working immune systems.

I'd figure between fungal and bacterial infections you would be incredibly ill within a matter of hours, and suffer respiratory collapse within 12 or 24 hours. You'd probably die from that before you went blind from dry eyes, as your tear ducts are clogged by other bacterial/fungal infections.

One upside is as you have no immune system there would be no pus weeping from your eyes.

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Five years ago, my father had a compromised badly-working immune system due to acute leukemia. An infection from some bacteria that was incubating inside his body caused septicemia and killed him in just a few hours even with heavy medical caring.

Not having an immune system is very quickly fatal except if you somehow manage to live your entire life inside of a sterilized bubble or something like that. Otherwise, every people has a lot of bacteria, viruses, protozoarians and/or parasites inside his/her body that could kill in only a few hours if the immune system doesn't keep them controlled.

Most of those pathogens act by killing healthy cells, producing toxins and/or damaging tissues. Further, the immune system needs some time to combat an infection. Also, the greatest the infection and the weaker the patient is, the less is the chance for eventual recovery and the tougher is the challenge for the immune system. So, if a patient without an immune system is already severely ill and suddenly, the immune system comes back, he/she might already be unsavable.

So...

  1. What length of time would be immediately or almost immediately fatal? (so even restoring it completely would not save from a quick death)

This, of course, varies from person to person. But I assure that many healthy people would be dead in less than 24 hours if the immune system suddenly vanishes.

  1. What length of time would be survivable but lead to long term (or permanent) consequences?

  2. What length of time would be completely healable afterward, even without modern medical treatment?

There is a smooth gradient and probabilities in the game ranging from certain death to unnoticeable. Also, again, this strongly varies from person to person. So there is no place to draw any hard line.

  1. What length of time would be unnoticed or almost unnoticed?

Probably only a few minutes. Maybe up to 2 hours.

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  • $\begingroup$ I'm sorry for your loss Victor, thank you for sharing your Dads story with us. $\endgroup$ – Binary Worrier Oct 7 at 7:11
  • $\begingroup$ @BinaryWorrier Yeah. That was a very difficult and sad time. $\endgroup$ – Victor Stafusa Oct 9 at 2:10
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Cook a porkchop and leave it out of the fridge for a while, noting the time it takes to spoil. In my experience, half a day would be enough.

Without an immune system, you'll have about as much defense against bacteria as a porkchop.

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  • $\begingroup$ Should it be a recently-died carcass instead of porkchop? $\endgroup$ – Akangka Oct 1 at 11:55
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    $\begingroup$ @Akangka The problem with a recently died carcass is that you have to wait until all immune system cells die, at which point it's not as recently died anymore. $\endgroup$ – Dmitry Grigoryev Oct 1 at 11:58
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    $\begingroup$ You're right, but a cooked porkchop has a very different chemical makeup (from heating and seasoning). Comparing it to live flesh is basically an apple to an orange, then. $\endgroup$ – Akangka Oct 1 at 12:02
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Human blood already contains lots of antibodies that are memories of the past diseases, these would stay where they are unless destroyed by some really super-method. A newborn has enough antibodies for about six months from the mother. I think the adult human may survive for a comparable duration if just the mechanism producing new antibodies is destroyed.

There are also defenses like acid in the stomach and lysozyme in the eyes and lungs. If they stay working, this would help a lot. It definitely will not be minutes.

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