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Hal Clement's novel Needle was revolutionary in featuring a non-parasitic alien lifeform capable of inhabiting a human body.

Specifically, the Hunter in its independent form is a gelatinous mass capable of amoeboid swimming and slow crawling like a slime mold. It is implied (though I do not recall if it is explicitly stated) that the creature's body need not remain contiguous, and it can separate into a swarm of individual cells circulating through its host's body without losing its identity. Despite this inconstant form, they demonstrate human-level intelligence. Creatures of the Hunter's species can seep through the skin of humans and other animals to enter or abandon hosts, and can choose to take over their host's nervous systems, or hide. Over time, they will destroy the host immune system to preserve themselves, but take over its functions so as not to leave the host vulnerable. When in humans or other sapient hosts, they typically wiretap sensory nerves, and communicate with the host by drawing shapes with their bodies on the host's retinas.

To what extent is a creature like this actually plausible, and for those features which are plausible, how would they accomplished?

A list of all of the Anatomically Correct questions can be found here: Anatomically Correct Series

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Unfortunately, this alien lifeform is only plausible for a very specific value of plausible, namely "almost completely not plausible". What makes this being implausible?

Firstly, its ability to enter a human body unnoticed. Humans have skins which are designed to prevent most commonly found substances from penetrating, and those things that do get through the skin are either chemical in nature, and therefore much smaller than anything as complex as a biological entity, or they are macroscopic, and their entry causes a noticeable injury. For this being to be able to penetrate a human's skin unnoticed, its cells would have to be very much smaller than our own.

Secondly, the being's ability to dissociate while retaining its intellectual capabilities is highly implausible. Intelligence is a result of high complexity, and dissociation would result in a reduction in complexity... unless the individual parts were able to communicate with specific other parts without requiring a physical connection. It's difficult to conceive how and why such a means of communication would even evolve, let alone work within a human body. Should the alien's cells have evolved to communicate via non-contact means, this would likely increase their individual complexity and size, making a covert infiltration into a human less likely.

Thirdly, the alien is going up against the human immune system, a system that will most likely either not recognise the alien as a threat, or will regard it as a large and widespread threat. In the former case, there is no need for the alien to disable the human immune system, but in either case, disabling the immune system is a difficult proposition, as the alien neither knows how intellectually, nor has evolved to be able to do this instinctively. In the event that the alien is recognised as a threat by the human immune system, even if it prevails, the human host is likely to display many symptoms of illness that will make a covert takeover a virtual impossibility. Finally, if the alien does manage to neutralise the human's immune system and substitute its own, the likely differences between the two would most likely result in things that the human immune system would have recognised as a threat being ignored, and things that the human immune system does not recognise as being a threat incorrectly being attacked as a threat, resulting in bouts of sickness from common opportunistic pathogens, new, unexplained allergies and autoimmune diseases.

Fourthly, performing a wiretap on human nerves would require a considerable degree of intelligence, along with a considerable amount of trial and error, and could conceivably result in the introduction of spurious inputs, resulting in phenomena such as unexplained random sensory and motor tics, and/or numbness and/or paralysis, along with disturbances of the sympathetic and parasympathetic nervous systems, which could result in symptoms similar to those of a disease, up to and including death.

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Sure, you can do this!

There seem to be a couple major points of concern, which I'll go over in detail below: swarm behavior, infection process, immune system interactions, and sentience.

Swarm behavior

The Hunter is described as a mass of cells, able to separate into individual ones or work together as a colony. This is not unusual behavior for life: there are many examples of colonial organisms here on Earth, from the simplest choanoflagellates and slime molds to the highly specialized and complex siphonophores. This last group is of special interest is the question because the well-known Portugese Man 'O War belongs to this group and is therefore not a true jellyfish, but instead a colonial organism:

Image of a portugese man-of-war, from http://mentalfloss.com/article/550892/facts-about-portuguese-man-o-war

Given that the Hunter is described as resembling a four pound green jellyfish, I think we've found a winner. Admittedly, the man-of-war is unable to separate completely into individual cells and remain highly functional, but Clement's novel also fails to discuss the specifics of the Hunter's decomposition and reassembly, so it's entirely possible that there's a minimum cell number that the alien species must maintain to stay alive.

Infection

Infection is actually one of the simpler steps in this process. Assuming a colonial existence as detailed above, the cells themselves would likely share many traits with other amoeboid cells, such as the body's own leukocytes, better known as white blood cells. These cells are tasked with hunting down and finding bacteria and other invaders in the body and destroying them via phagocytosis, as seen in the linked video.

What's cool about white blood cells is that they move with surprising freedom throughout the body, given their relatively large size. The amoeboid form actually allows them to squeeze through tiny gaps in capillary walls to respond to damage:

white blood cells migrating through blood vessels

Now, human skin has a higher cell density than internal cells and the especially porous blood vessels, but an alien life form could also have a different internal setup that allows it to squeeze through smaller spaces. In fact, it's likely that this trait would be heavily selected for in a symbiotic species where internalization is hugely important.

Additionally, leukocytes already perform complex swarm-forming behavior, further validating problem #1 above.

Immune system interactions

This is a trickier problem, partially because there's only so much we know about the human immune system. The human immune system is composed of a large consortium of cells vaguely broken down into innate immunity and adaptive immunity. The more pressing of these is the innate immune system, as it's the one that deals within initial invasion and body response. The Hunter would essentially have to evade detection by either tricking the MHC complexes or by destroying the cells upon contact. This is also something that the Hunter could easily take over the role of in the body, as the main function is to find new things and destroy them.

The adaptive immune system is a different kind of problem. This isn't so much an immediate threat to the alien because it's designed to respond to repeated attacks by the same pathogen, but it would be difficult to take on the role of because this system acts as a kind of cell "memory" for pathogens. The B-cells of the adaptive immune system store antigens for specific bacteria and viruses that the host has already encountered, and transferring that information to the symbiote would be tricky as we understand it. The host would essentially stop being immune to any prior infection, and any vaccines would need to be re-administered.

Sentience

It's clear that the Hunter possesses true sentience, rather than simply high levels of intelligence, because it's able to learn and communicate complex ideas to the host. This is a tricky one to reality-check because we know very little about what establishes sentience and what separates an internal mind from a behavioristic response. As far as we know, intelligence and sentience always requires neurons of some kind - perhaps the alien species has an analog for these information-communicating cells or something weirder that works on a per-cell basis, given the colonial nature outlined above.

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The biggest problem, to my mind, is the intelligence of the thing. Being able to aggregate into a superorganism capable of intelligence... well, that's scifi, but you could see how it might be possible. Being able to disaggregate again, yep, that's fine. Being able to remain sentient, or at the very least, goal-oriented whilst disaggregated? Yeah, no.

Thought is clearly very complex. Our brains are phenomenally complex things which are formed by an extremely dense network of interconnected nerve cells. You've got about $10^{11}$ neurons in your brain, but each of those neurons is connected to an average of 7000 neighbours. Its that interconnection that enables your brain to do amazing things like discover fermentation or invent arguing on the internet. That disaggregated blob of cells has no such physical networking.

That's not to say that communication between parts of the cloud of cells is impossible. You could perhaps imagine shedding specially tagged proteins or viruses into the host body, such that only specific cells would absorb and react to their payloads. A kind of communications protocol encoded in amino acids or nucleic acids, with sender and receiver identifiers. The problem there is that the spreading of those communications packets is slow. It could take minutes or hours for the target cell to receive a specific message, and then minutes or hours again for it to send a response, even assuming the message doesn't get damaged, excreted or eaten by something else in the mean time. Compare that with nerve conduction speeds, over 100m/s. Your disaggregated swarm might be able to think, but it would do so glacially slowly.


What you need, then, is something that can form a network. My model for this would be more like a fungus or parasitic plant, capable of forming haustoria (for fungi, a specialist form of hypha, in plants a specialised root). Fungi are sessile, so clearly the sort of organism you need is something slightly different. Slime moulds are interesting, having some form of motility, but they aren't fungi. Something a little like a combination of the two might work.

Fungi can spread as spores, or perhaps by contact with an existing fungal network. They already have the means to invade host bodies, which would only be enhanced by careful immune modulation. Trying to "seep through the skin" instead of tunnelling in network threads like this is probably impractical. Motile slime-mould or amoeboid forms could get in via some other means. Naegleria fowleri gets to your brain via your nose, but the human body is well equipped with plenty of soft entrances and exposed mucous membranes... you have so many to choose from!

Infection via spores or small motile forms has a second problem with disaggregation: remembering things. A single amoeba isn't going to be able to think very hard. It does have the ability to store a good few gigabytes of information, in the form of DNA. An extra set of chromosomes or plasmids storing memories instead of being a normal part of cellular operation would clearly be required. To successfully infect a new host, many spores or infection cells would be needed to contain the whole of the organism's mind. The dumb infection vector grows and spreads and hooks up a network until it becomes big enough to think and unlock the stored information in all those DNA databases.


Evading the immune system is the second hardest issue; doing so without exposing the host to opportunistic infections and having them die on you just as you got settled in would be tricky, to say the least. Learning all of the "knowledge" encoded in the host's adaptive immune system and replicating it would be extremely non-trivial.

The best approach would be to simply not mess with that. Use an infection form capable of resisting or evading the immune system (there are lots of models for that... mycobacteria for one, HIV for another or even one of the many herpes-type viruses, but there are all sorts of intracellular parasites to take your inspiration from. Making a general purpose immune-evading strategy is probably impossible; you'd need to use your brains to tailor your attack to each new species. Just make sure that you can make your own cells (or infected host cells) produce the right kind of surface antigens that keep the host immune system happy. You could always put up with the host being super ill during the infection process... either as a result of immune reaction to your invading cells, or as a result of immune suppression allowing opportunistic infections, but just long enough for you to establish how to trick the host immune system at which point the reaction stops and they get better. Sure, weaker hosts may die, but tough hosts are the best hosts, right?

Remember, you can't make an omelette without breaking a few eggs! Maybe a whole load of humans have to die or go mad before you've perfected your technique, but you'll get there eventually. Their deaths aren't implied to kill the infecting organism too, so write up all your notes about what went well during the infection, and what didn't go so well, then disaggregate and go find some new hosts to try again on and hopefully do a better job next time.

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