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Just for clarification: when I say "designing", I mean just coming up with the idea of this fungus, as opposed to it being "designed" by humans.

I've been very interested in fungi ever since finding out about the cordyceps fungus on The Last of Us and The Girl with all the Gifts. The idea that a naturally-occurring plant can essentially mind-control insects is fascinating and horrifying in equal measure.

So I was wondering - what would be the features of a cordyceps-like fungus that would allow for it to actually be symbiotic with humans, i.e. what would its biological mechanics/features be like? It would have to have the following features:

  • Releases spores, just like the cordyceps fungi
  • The spores have an extremely high mortality rate - kills 90% of people it comes in contact with at first
  • For the others, let's say that they don't kill the people it comes in contact with, but rather creates a mutual symbiotic relationship
  • Maybe the spores that the fungus now produces can be more "targeted" - i.e. it can specifically aim to infect more healthy humans to use as a symbiotic "host" as opposed to killing them
  • Compels the "infected" to seek out healthy humans - this is their compromise for having this symbiotic relationship with the fungus

How would such a fungus hypothetically work? Obviously nothing like this exists in the real world that we know of (at least not to this level of control over humans).

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    $\begingroup$ Just a thought, "symbiotic" and "high mortality rate" are basically opposite concepts entirely. At that point, the best symbiotic relationship it can offer is immunity... $\endgroup$ – krflol Oct 21 '19 at 16:21
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    $\begingroup$ It hypothetically works like a cordyceps species, only in a more complex and nuanced way. What more did you want? What makes one answer to this question better than another? $\endgroup$ – Starfish Prime Oct 21 '19 at 16:25
  • $\begingroup$ What Plague Inc has taught me is that you get the fungus to only start spreading, without causing any symptoms first, then once enough of the population is infected, start rapidly mutating and destroy all infected while infecting even more people. $\endgroup$ – VLAZ Oct 21 '19 at 16:29
  • $\begingroup$ @krflol this is very hand-wavey, but maybe it starts off with a high mortality rate, but eventually, the symbiotic relationship forms when it turns out that it's more efficient for the infected to directly target uninfected hosts as opposed to them dying and the spores flying around willy nilly? Perhaps the fungus can manipulate body chemistry of the infected people to reduce pain and heighten adrenaline, making them more effective "spreaders" of the spores. $\endgroup$ – Space_Cadet Oct 21 '19 at 16:37
  • $\begingroup$ 'to reduce pain and heighten adrenaline' That sounds more like rabies than a symbiotic relationship. Instead, what if your fungus allows humans to digest different types of food, like leaves or grass? Or, what if it replaces and upgrades the immune system (bonus: this would explain the 90% mortality rate, as a lot of people would die during the "upgrade"). $\endgroup$ – alexgbelov Oct 22 '19 at 0:29
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Start from a common fungus that infects a lot of people - say, malassezia or any of the fungi that cause Athlete's foot.

Now just have the fungus produce prions that make it to the blood of the human host. It might take from weeks to decades, but the hosts will develop symptoms like those of kuru and mad cow. The latter may get violent. A majority of all the infected will die in short time (three to twenty-four months), but some may last longer. Those who last AND get violent might have a behavioural change which causes them to wish to lash at, scratch and bite anyone - should they do that to a healthy person, there is chance of infection. Since we are talking about the kinds of fungi I suggest, handling the infected of their clothing, or just conviving with them for even a few days might be enough to spread the disease even without the need for a bite or scratch - these [redacted] leave spores eveywhere. It's just that hitting a wound would accellerate the process.

You wanted a symbiont, so what could these fungi do for us? Well, fungi are in constant war against bacteria. If these fungi secrete penicillin (which happens to be a fungal product), then the infected would be more resistant to... Whatever bacteria haven't evolved a resistance to it yet. Well, I'd rather die from a brain-eating fungus than from a flesh-eating bacteria such as syphilis!

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    $\begingroup$ I was all interested with your starting paragraph. An epidemic of mind-controlling athlete's foot might be correlated with a sudden uptick in runners and running clubs, businesses catering to runners and apps related to performance tracking and sharing. Who'd have thought that parkrun would be a harbinger of the sweaty, itchy apocalypse? Then the bit about prions took it all a bit downhill. Oh well :-( $\endgroup$ – Starfish Prime Oct 21 '19 at 16:36
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Fungi do cannot directly change the way your mind works, they are way too primitive for that. Fungi can change the chemicals and hormones, and those affect the way the mind work. So you are looking for basic human instincts that fungi can amplify:

Aggression instinct

Fungi could make the person more aggressive, and also stronger and harder to hurt. Only the most physically fit can survive the transformation. New host gets infected through skin cuts or bites from an infected individual.

Sexual instinct

Fungus is transmitted sexually, including oral sex. Fungus makes the infected victim seek sex with many partners. Fungus does not need to make the victim look more sexy, it just creates the urge, and humans use their own brains to come up with ways to meet it. Women will dress and act slutty, men will hire prostitutes or rape. The 90% kill rate will get the fungus discovered pretty soon; to let it spread longer, give it a long incubation period, or a 10% death rate.

Caring instinct (Oxytocin hormone)

Oxytocin increases in happy couples, but also in good parents. It literally makes the person more caring and helpful. In particular, they are more than happy to share a certain food supplement pill that makes them feel so great and happy and energetic. Unlike MLM's or drugs, they do not charge much for it, and they get unlimited supply from their dealer, and they eventually discover that their own body produces the key ingredient of the pill. Again, you will need reduced or delayed lethality to make this work on large scale.

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Humans have a lot of tools at our disposal from buildings/HVAC to medicine so I think first and foremost this fungus has to encourage people to socialize and manage fear/logic in some way to make us less likely to effectively notice/combat it before it gains a foothold in society. Here's how I think you can make it work:

  • The fungus produces a chemical similar to safrole which is processed through the organism until eventually becoming MDMA in the spores. Breathing in the spores starts microdosing the subject with a pure form of Ecstasy making them happy, be active, socialize, get into large groups. A wave of energetic pop-up gatherings fueled by social media exposure will lead to huge participation of uninfected.

  • The fungus kills by overwhelming the lungs, towards the end of a subject's life they are violently coughing and spreading psycho-active spores, if you want something more sensational you could even go with spore development continues until the chest fully bursts like a puffball mushroom. Roughly 10% of the populace has biome conditions (pH, antibodies, etc) such that the fungal growth is restricted/fought back and can't overwhelm the body instead finds a balance with it.

  • Before people are wise to its lethality the primary wave of infected will be some of the society's most fit, the rich, social influencers, etc (think everyone who went to Fyre Fest and anyone of their followers who'd do a meet and greet). Not only does this allow for rapidly spreading the infection, but a significant hit to herd immunity as those who's bodies would fight the most and take the longest to become overwhelmed have largely been affected.

  • The psycho-active spores also provides possible explanation for why this exists, it was designed as a way of creating pharmaceuticals either illicit or legit. Maybe there was no reason to suspect that it would grow in human bodies (like how we can eat cordyceps without a problem), the fact it does is just a tragic coincidence. Since being around it makes you at least mildly euphoric its handlers never suspected an outbreak until after the deaths were investigated and understood.

  • Lastly since it manufactures chemicals intended for human interaction, the first known of which is habit forming and alters emotional and neuro-chemical response you have a good footing for explaining just about any form of effects for the people that don't die as well as why they are more likely to appreciate the fungus despite the fact that it killed everyone they knew and loved. Maybe 15-20% establish a balance with the fungus but half of them still die in the aftermath of the infection (suicide, looting, collapse of society, etc).

This concept has the added bonus that many people simultaneously idolize and despise the people subject to the first wave, so knowing that the like of the Kardashians and Jeffree Star are the infection vector makes it easy to believe it could spread incredibly fast while satisfying that part of us that thinks they are dooming our society or at least wants to watch them fall.

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    $\begingroup$ Nice first post, detailed - answered the question. +1 (From review). $\endgroup$ – BLT-Bub Oct 22 '19 at 23:27
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There are a number of pathogens that have significant influence on host behavior in ways which are frankly almost as disturbing as cordyceps.

Gut bacteria

Human gut bacteria have been known to release chemicals similar to human hormones in response to preferred food sources to encourage their hosts to eat things which encourage them to grow.

Toxoplasmosis

Affects mice's inhibitions to being out in open and makes them less sensitive to odor from predators, making them far more likely to be eaten by cats. It has also been shown to increase impulsiveness in humans (particularly those of us with RH- blood types)

Dracunculiasis

Possibly the fiery serpents from Exodus and the source of caduceus symbol. Ick, just ick. Water transmitted, and causes a burning sensation in the skin.

That said

With these examples, modification of the endocrine system to encourage any sort of behavior is well within the realm of believability, unusually hungry, thirsty, aroused. It could even increase concentration. Many drugs are derived from fungal sources.

If you are looking for some mutually beneficial effect humans have some issues when cortisol levels are elevated for extended periods of time. While short term boosts increase healing, tamp down immune responses, increase oxygen availability, it leads to diabetes, obesity, heart disease. If the fungus helps humans thread the needle you could almost have supermen and women.

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Zombie fungus.

I know, I know! Done so much! But here is how it would work. No hard science tag so this is just storytelling.

Fungi infect a chordate. They do that all the time. These fungi are symbiotic in that they augment healing and disease resistance. Infected folks who are hurt bounce back quick. Sometimes they regeneratively bounce back farther than where they started, so to speak. Unfortunately most infected folks also get encephalopathic as a side effects of the regenerative abilities conferred by the fungus. They are confused, possibly over emotional, possibly aggressive - it varies from individual to individual. Many of these encephalopathic infected die as a result of poor decision making or inability to meet basic needs, like food and water. Some get by ok, living like animals or cared for by other humans.

Some infected (10% as per OP) are encephalopathic to a lesser degree, or not at all. Occasionally the mental acuity of an infected person is better than it was before infection. These people have the regenerative powers and disease resistance of any other infected person but are in command of their faculties and so can do well, or even better.

It can be a spectrum, which is more realistic and offers more story telling latitude.

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The scenario for this would be in the context of some other existential threat for the humans. Perhaps the fungus was released by would-be alien colonists. Your protagonists, in the course of dealing with this threat, must also deal with the changes to their own bodies and minds and with changes in the people around them and society as a whole.

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