I have conceptualized a disease that rapidly kills cells in the human body and replaces them with modified copies over a period of 7-10 days. After this time, every cell in the infected person's body should be replaced with altered equivalents that confer biological immortality. Since the "disease" is actually caused by a certain entity's biology-warping powers, the immune system is unable to protect against it.

My question is this: What would actually happen to somebody whose cells were rapidly dying and being replaced like this? I'm afraid that the results might be catastrophic, and I'm not sure it would even be possible to survive the process. My current most dire estimate is that the symptoms might resemble body-wide necrosis in every organ, or extensive radiation burns throughout the body's tissues.

I am hoping that the reality is not so severe as this, since I can at most afford for the external symptoms to be about as severe as a mid-level sunburn (and preferably not detectable at all). But if that isn't realistic, I may have to rethink the timeframe of the infection or invent an alternative.

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    $\begingroup$ If there's no more handwaving, this will require the subject consume their entire body's worth of energy and then expel their entire body's worth of mass. This seems grossly untenable to me, the human body isn't built for that kind of abuse. $\endgroup$
    – Halfthawed
    Commented Jun 27, 2022 at 19:16
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    $\begingroup$ The question is where does the dead cell go. If it simply pops out of existence you are good. If it hangs around to get disposed of the normal way then you should look into the normal way to see how THAT works. $\endgroup$
    – Daron
    Commented Jun 27, 2022 at 19:25
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    $\begingroup$ Why kill the cells causing necrosis and toxic degradation, why not modify them? Instead of killing and removing the whole cell, replace its components molecule by molecule. If the process is controlled so that function is preserved, it might leave even sensitive organs like the brain relatively undamaged. $\endgroup$
    – Cloudberry
    Commented Jun 27, 2022 at 19:56
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    $\begingroup$ There are some recent cancer treatments that enable the immune system to target the cancer cells. Some of these have been too successful in terms of the cancer cells being killed so rapidly it overwhelms the liver and kidneys. So say 1% of body mass in 24 hours? Additionally that much activity will cause heating/fever. which can cause mutations, I suspect timeline of 7-10 days is much too short. $\endgroup$ Commented Jun 27, 2022 at 23:11
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    $\begingroup$ Why kill and replace the cell? Could you replace just the nucleus, or add certain genes to the nucleus, or add an organelle to the cytoplasm that does whatever you need done? There are many researchers who believe that just changing a few proteins or biochemicals in a cell can allow immortality or something closer to it at least. Look up David Sinclair for an example (I'm not offering any opinion on this, just encouraging you to look). $\endgroup$ Commented Jun 28, 2022 at 0:59

5 Answers 5


Bootstrap to immortality

@Halfthawed raises a fascinating point that I'd like to run with.

The concern is: since it sounds like this is all being accomplished via the body's normal mechanisms, you'll need to take into account what it looks like when all those mechanisms run at ludicrous speed™️. That means: each cell will require a ton of energy and nutrients, and produce a lot of waste. Presumably it's the same amount that occur during normal replacement, but what's special in this case is that every cell is doing this near-simultaneously.

Following that train of thought, I'd expect that the kidneys will be put through hell trying to filter that much waste from the bloodstream in such a short time. Maybe that's fatal, maybe not. Other systems will face similar challenges.

So I think your entity will recognize that it needs to bootstrap this process: upgrade specific organs in a particular order to accommodate subsequent steps of the metamorphosis.

I can't tell you what order is best: perhaps gut + kidneys, then heart, then lungs, then liver & pancreas... I really don't know. But, I think that is what will dictate the symptoms, and so it will present as a kind of roving malaise accompanied throughout by significantly increased appetite and diarrhea.

Another way to accomplish this would be grow additional organs temporarily which are designed to bridge the gap, and which get absorbed back into the body after the metamorphosis is complete. That would probably manifest itself as a tremendous appetite during growth, and possibly some mild physical discomfort that's hard to pin down because the body is now stuffed full of organs. Then during the metamorphosis proper, that part of the body would probably be unusually warm because of all the blood flowing through it.

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    $\begingroup$ If the cell death occurs through apoptosis, wouldn't the body just consume the dead cells and use them to help make the new cells? Or would the sheer number of dying cells just overwhelm the body's systems that handle apoptosis? $\endgroup$
    – Skallod
    Commented Jun 27, 2022 at 23:28
  • $\begingroup$ For some reason, 'grow additional organs' is bringing the image of a bloody cocoon to my mind. $\endgroup$ Commented Jun 28, 2022 at 7:12

rapidly kills cells in the human body and replaces them with modified copies over a period of 7-10 days. After this time, every cell in the infected person's body should be replaced with altered equivalents that confer biological immortality.

This is cancer.

The modified copies of cells are modified to enable them to reproduce quickly and not die. They are malignant cancer cells. Through a variety of methods the proliferating modified cells starve out or otherwise kill the native cells. The problems is that the modified cells lack the specialized functions of the cells in organs that keep the whole organism alive and when these organs fail the modified cells die with the organism.

Unless they escape the organism. Hela cells for example are such cells, and these are alive and well in the world. These immortal cells escaped their host 70 years ago and are maintained artificially in many sites around the world. Hela cells escaped with help but transmissible cancers escape and propagate from host to host on their own - transmissible clam cancer being a frighteningly weird example.


I'm no biologist but I will answer your question to the best of my ability.

In short:

Because cells are growing faster, you will not only produce friendly cells but also dangerous cells. Your immune system might not be able to fight those dangerous cells off quick enough. And that way you might get diseases. (e.g. Cancer)

Long 'explanation'

As long as the overall rate of cellular biosynthesis (production of biomolecules or anabolism) is greater than the overall rate of cellular degradation. You should be fine. Though keep in mind, that normal cells get 'corrupted' sometimes, which means that your body sometimes just screws up. If that happens the body might make cells that aren't good for you (for example cancer.) Normally you wouldn't have to worry about that. Because your body can defend itself against that, But if too many of these 'corrupted' cells are created the disease might grow and your body will not be able to defend itself.

Now Because cells are growing really fast in this scenario 'corrupted' cells are created much faster. Therefore your immune system would need to work faster and harder, Most humans would not be able to do that.

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    $\begingroup$ The cancer concern is on-point, but I suspect it's reasonable to ignore in this scenario because it sounds like the whole-body mutation is being caused deliberately by some entity, and I get the impression that entity can monitor the process, which would allow it correct any missteps. But, perhaps I have misread between the lines, and the technique does not give fine-grained knowledge or control, in which case cancer would be a major risk. $\endgroup$
    – Tom
    Commented Jun 27, 2022 at 19:35

If the new cells are advanced enough to both perfectly mimic the function of the old cells, and slurp up the remains of the dead ones so that it doesn't have to be excreted through the body's normal cleanup systems and could, instead be used to fuel further transformation, then theoretically this could be accomplished with little outward sign at all.

Of course, a cell type that advanced would probably confer far more than just biological immortality, first subverting and replacing the immune system with something amenable to resculpting the body, and then replacing everything with a maximally functional copy... Kind of like the concept behind Captain America. And the intelligence level of the genetic programming necessary to pull this off would also open the door to rapid healing, among other things. And if you're going to mess with someone like this, not doing all that while you're at it should, at least, draw a little criticism from any other deities/angels/sufficiently-advanced-technologists for a waste of an opportunity.

Alternatively, since your goal is biological immortality, there's not much reason to replace the cells at faster than their normal replacement rate. This process would take years, but so what? All the tissues with fast turnover tend to be the ones which start to show signs of age first, and they'd be replaced quickly. And if it takes ten years for the last scraps of bone to be replaced... Who's going to notice? Inject a few immortal stem cells and make them a tiny bit more aggressive about moving in and taking over and the person's outward appearance of aging will effectively stop within a couple weeks, and fully stop for practical purposes within a year or two.

Your third option is to just do it fast enough that the person calls in sick, isn't responsive for a couple days, and then is back like nothing happened. Harden the skin into a chrysalis so nothing leaks out when you melt everything but the brain, heart, and lungs into raw, nutrient soup for a while, and they just feel icky, take some cold medicine, fall asleep, and wake up three days later hungry and dehydrated but seemingly fine with no memory of what happened aside from fading nightmares.


You Would Be Better Off Using Machinery

If you are wanting to avoid hand-waving of unbelievable amounts of nasty puss and other necrotic and biological waste materials plus a ridiculous amount of fever heat, you need to have this handled by machinery.

This process requires a great deal of energy input to rearrange DNA, create corresponding body systems for long-term structural maintenance and repair as well as handle the immense amount of energy needed to fund the transformation plus the waste heat.

That machinery could be an enclosing coffin-like affair, nano-tech or something like a transporter that analyzes the person at the molecular level, transmutes them to energy and then back again to matter in the new form. Regardless, it's a lot less, well, yucky, than the alternative.

As others have suggested, a cocoon would be a way to do the equivalent of the first machine in biological form but it still needs to provide for mass energy input and waste heat disposal somehow. Maybe a dragon-like creature that eats people and excretes the new version?

To poop thou becomest and from poop thou returnest?


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