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So I have a brain parasite that needs to enter the skull of a host organism (in this case humans) with the least amount of effort, but is too large to enter through the circulatory system and must physically crawl inside a host. Drilling through the skull is the most difficult option and will likely cause fatal infections. This is fine for brain parasitoids that eat the host from the inside out, but not for parasites that need the host alive for longer periods. Going through the ear canal still requires destroying the middle and inner ear which would cause deafness and loss of balance in addition to still drilling through the skull and causing fatal infections. At this point the most efficient and least destructive options seem to be to enter through the nostril/nasal cavity or eye socket where the brain casing is thinnest or absent and drenched in insulating mucus.

What option makes the most sense?

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  • $\begingroup$ How big is the brain parasite? Is it 'swidgey' like a slug, (or octopus), or hard and crunchy like a beatle? $\endgroup$ – Ewan Jul 23 '16 at 19:50
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    $\begingroup$ Seems like the nose is the obvious solution, not sure what you thought other people might suggest ... $\endgroup$ – AndreiROM Jul 23 '16 at 19:52
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    $\begingroup$ this reminds me of the Animorphs series… $\endgroup$ – taylor swift Jul 24 '16 at 4:22
  • $\begingroup$ This is parasitology, but not as we known it. A para-parasite, in the words of Brian Aldiss, that behaves quite unlike real-world parasites which are usually ingested as eggs or in a larval form. If eggs, they hatch and the parasite either travels via the circulatory or bores its way through the host's flesh and organs until it reaches its target organ, in this case, the brain. $\endgroup$ – a4android Jul 24 '16 at 6:13
  • $\begingroup$ What if the parasite is a virus of the mind, a meme? It enters the skull by being learned, and does not have physical form. $\endgroup$ – JDługosz Jul 25 '16 at 10:27
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You have your standard entry points, mouth, nose, eyes and yes ears. But I have an alternative natural entry point for you.

How big is your parasite? Can you not have it start small and grow, transform, mutate into something bigger when it gets to the brain?

Regardless of the size, you do have another hole in your skull. Completely natural, no drilling required. Ever heard of the foramen magnum? It's the hole in the base of the skull through which your spinal column passes.

If your parasite is small enough it might be able to squeeze through past all the spinal cords, and nerves, juicy niceness and enter the brain, free as a bird.

You can then have your parasite enter your body in any manner you can think off. It can then physically crawl under you skin, through the muscle (sounds freaky and painful) until it reaches the spine. Follow the spine upwards until it hits the brainstem. This will a congested area, as all the necessary muscles, nerves, veins cartilage etc tries to fit into a small tunnel area. It will be a squeeze and probably cause all sorts of (temporary?) pain, fits, spasms, paralysis but it could crawl through without having to 'drill'.

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  • $\begingroup$ Like The Host, but with more side affects. $\endgroup$ – Xandar The Zenon Aug 9 '16 at 22:25
  • $\begingroup$ Wouldn't know. Havent got around to reading it and I'm pretty sure the movie didn't do the book justice! $\endgroup$ – EveryBitHelps Aug 10 '16 at 5:59
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    $\begingroup$ Well, I'm a human male and therefore I disliked the book. I was told afterward it is like a chick flick in book form $\endgroup$ – Xandar The Zenon Aug 10 '16 at 6:01
  • $\begingroup$ Depending on the two different morphs of the parasite it might not be a squeeze at all. $\endgroup$ – Joe Bloggs Sep 20 '17 at 6:03
  • $\begingroup$ Wait, _The Host_was like a chick flick in book form? Also, why would it make sense that you disliked it as a human male? $\endgroup$ – Alendyias Jan 3 at 0:10
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Your notion of drilling through the skull as being negative is false. Brain surgery has been around since the Stone Age. There were even high success rates circa 7000 BC. There is no reason for a parasite not to be able to drill through the skull into the brain. It could even secrete a anti bacterial/viral mucous to prevent infections to prevent injury to the host.

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You could leave the majority of your parasite's mass in the flesh of the neck or abdomen and have it send tendrils up into the brain through any of the openings described in the other answers.

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How about this: the parent of the parasite enters the body through an orifice, then injects its larvae into the blood and dies. The larva passes through the blood, matures in the brain and it looks as though it is the same creature as the parent, even though it is actually the child. When the host dies, the parasite crawls out and seeks out a new host. This is small potatoes compared to the complexity of some parasites' life cycle.

As for which orifice, it doesn't matter too much. Going into the mouth and passing through the digestive system offers the benefit of allowing the parent to dissolve without anyone noticing. Perhaps the parent is not resistant to digestive juices, but is filled with eggs that are - once the parent is digested, the eggs are released and burrow into the bloodstream through the large intestine.

Maybe while the organism is in the brain, it is actually a colony meshed together (this works better if the organism's shape is fairly simple in appearance, like a blob or a slug as opposed to an insect) and then when it leaves the brain, it rips open the 'bag' and all of the smaller organisms crawl away.

Alternatively, it can have multiple castes and a generational cycle - outside a host body it reproduces sexually, allowing it to produces multiple eggs that hatch into the orifice-invading caste; this caste then produces the brain-invading caste inside the host's body through parthenogenesis, and uses its control over the host to seek out other infected hosts before dying and allowing the parasite to escape and breed.

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Regardless of where the entry point ends up being, you could give the parasite traits of leaches and maggots, and have them secrete numbing agents as well as cleaning the wounds to prevent infection. They could also even secrete antibiotics and antivirals to keep their host in good health.

For entry to the skull, my suggestion is a variation on the nasal passages. Initial invasion is through the circulatory system. When passing through the lungs, it moves into the airways. The subject coughs the parasite up into its throat and climbs into the nasal passageways.

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Based on my coincidental, light knowledge of Transorbital Lobotomies, I would suggest the eye sockets (in line with Mantis Toboggan's suggestion). In a lobotomy, a long metal spike would be slipped in around a patient's eyeball, then used to punch through the back of the eye socket, where it would carve an arc through the patient's brain matter. This procedure would then be repeated with the patient's other eye socket. Note that this procedure was designed to be performed by staff at mental institutions, who would not have direct access to dedicated operating rooms or trained surgeons. But anyway, my point is that we can infer two things from this: 1 that the bone at the back of the eye sockets is very thin and easy to break through, and 2. people can survive and have survived having it punctured, without any form of remedial surgery or really any sort of medical treatment at all, and all without serious damage to the eyes themselves.

Consider looking in to other forms of brain surgery for ideas!

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Not knowing the physical characteristics of the parasite or how you intend for it to operate, I'd agree that the eye sockets and nasal cavity are the most viable options.

I particularly like the use of the eyes. It has that added spunk that comes with the potential of being, um, interestingly grotesque. That could be fun to work with, if that's what you're going for. Maybe do some research into how we've used the eye sockets for convenient access to surgery on the brain (lobotomies, etc.) to get some inspiration for how it could enter naturally.

Otherwise, the nasal cavity seems like your best, realistic bet.

Or, it could enter the circulatory system before fully developed.

Taking inspiration from my limited knowledge of the pork tapeworm from a sub-par biology course, perhaps the parasite itself does not directly enter the brain but "leaves the task" up to its offspring. Tapeworm larvae develop from eggs in the intestines and quickly enter the bloodstream, potentially resulting in Cysticercosis.

I don't believe we currently know how it passes through the blood-brain barrier.

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The best examples are from nature.

Hookworm is transmitted by the larvae burrowing into the feet (or other exposed skin) and entering the bloodstream. They get into the lungs, gets swallowed and get into the intestine.

Brainworms are transmitted by infected gastropods getting ingested along with leaves. The larvae then migrate into the central nervous system.

Pork tapeworm eggs are ingested and can migrate directly to the brain.

Edit: As pointed out in the comments, the OP said the parasite was too large to use the circulatory system. While I would assume that the larval form would be much smaller than the final parasite form, it would seem that the nose, into the frontal sinus into the brain would be the easiest path, followed by entry at the back of the neck. The parasite would then enter through the Foramen magnum. However, anything too large for the circulatory system is going to be large enough to be noticeable as it burrows in regardless of where.

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  • $\begingroup$ The question states that the parasite will be too large to use circulatory system and must crawl inside the host, so I'm afraid the examples are out of question. $\endgroup$ – Vylix Jun 30 '17 at 20:11
  • $\begingroup$ Would the parasite be too large in all stages of life? $\endgroup$ – Andrew Neely Jun 30 '17 at 20:45
  • $\begingroup$ Judging from the other answers, I think it does not necessarily big in all phase. However as you pointed out in the edit, if it is too big for circulatory system, then it will never fit in any other possible entrances, at least unnoticeably. It will be interesting if a beetle-sized parasite paralyze the victim temporaily and forcefully enter through mouth, nose, or ear. $\endgroup$ – Vylix Jun 30 '17 at 21:21
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A very small brain parasite that exists today is a prion

If you want to be truly insidious, attack your targets with one of those.

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