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For those who don't know, Hanahaki Disease is a fictional disease related to love. It can happen when someone has unrequited romantic feelings for someone, so flowers start growing inside them. The flowers get bigger, and the victim starts coughing them up. If the disease is at its final stages, the person coughs up entire flowers. You can cure it by surgery, but the victim loses the ability to love. The only way to end the disease is by death or if the love object returns the love and the victim is certain that the love is reciprocated.

Usually, the flowers are said to grow in the victim's heart and lungs, but I don't know if that would be possible without killing the victim really fast. Is it possible for the flowers to grow on the lungs or stomach in a way that doesn't make the victim extremely uncomfortable all the time and doesn't kill the victim fast?

Another good question is what would be a way that the victim gets the disease? It's a rare disease, so I don't think the answer would be it's contagious.

The answer I'm looking for is one that can make the disease anatomically possible, that kills the victim slowly, and can have a plausible way of being contracted, maybe curse related (my story is a medieval fantasy).

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    $\begingroup$ As usual, Nature likes to beat us to the punch here at Worldbuilding.SE, it's not a flower but with no-handwaving, genetic engineering or other kind of magic whatsoever, there is a case of a man who had a pea plant growing in his lung. Nature is weird... $\endgroup$ – Samwise Apr 15 at 23:53
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It would take an inordinate amount of hand-waving...

Let's start from the flowers. Can we make a flower develop in a human lung?

And the answer is no, we can't, there's not the necessary space. We can however imagine a flower-analogue that might: some sort of symbiotic/parasitic organism that is usually found on some ground-bound rodent, and grows on its skin. The organism has evolved to avoid the host's immune response and uses it for locomotion, providing some service in exchange - vitamin synthesis, maybe. When it's ready, it releases its pollen to the winds.

Normally the pollen will experience a brief flash of activity upon landing on the host's mucosae or the skin of a pup: it will quickly burrow to a safe place near the skin, and there lie dormant until the appropriate chemical cues will signal it's time to flower. This usually happens when the host is fully mature, in good health and the season is right. The organism flowers slowly and beautifully, sending pollen everywhere and keeping its host healthy to do it as long as possible.

If the host isn't in top shape, that's bad luck for the host and the parasite both. The parasite, as an emergency plan B to ensure its genes' survival, flowers as much as it can, dying in the process and damaging the host, to ensure at least one pollen wave is sent forth.

The pollen might be inhaled by a human being. Normally nothing happens because the chemistry isn't right: the pollen does not exit its dormancy phase, and when it does it dies soon after.

Nobody knows what triggers the disease because even people living next to the flowered rodents do not contract the disease very often. Indeed, most of them never do due to acquired immunity.

The notable exception is when the person is flooded by the right mixture of norepinephrin, endorphin and oxytocins, up to one full year after breathing the pollen (one year being the average time for a rodent to reach maturity). The hormone mixture, which tells a human's organism to start behaving as an unrequited lover's organism is expected to behave, also tells the parasite that conditions are ready to flower. And it happens so long after the pollen exposure, that the relation between cause and effect has escaped the medioeval natural philosophy of your world.

They would rather believe that only refined, cultured city dwellers (those farther from the rodents, and therefore with less immunity - if they knew what immunity was) had the required sensitivity to trigger the heartflower disease, while peasant love is just two animals in rut (this is not so unlike what happened in our own world; most (pre-)Stilnovist poets in the 1200's said as much).

Unfortunately the "flowers" don't agree with human bronchi, so they are quickly torn off and coughed up. The cycle repeats until either the human dies, or the hormone mixture is altered. The new mixture has again to be close enough, and that happens when the love is returned and, better still, consumed.

It is sometimes possible to excise the organism if it's near enough the top of the trachea. Those that have this done to them usually die of complications or infections (medioeval times being what they are). When they don't die, they almost invariably suffer from PTSD and unconsciously avoid the same situation - when they would start feeling in love again, they get flashbacks instead, which quickly kill the mood.

Otherwise, the flowers would continue to grow, and some pollen find its way throughout the circulatory system. This, plus some cases of vegetative endocarditis, led to the mistaken popular belief that flowers grew in the heart as well as in the lungs, whence their name, "heartflowers".

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A kind of fungal infection? (not-so-hard science ahead)

Some fungi can grow a fruiting body on their host. The most famous case is the Ophiocordyceps unilateralis (google it at your own risk), which also modifies the behaviour of the ant having the misfortune of being its host, by releasing some enzymes that alter its behaviour. The ant leaves its nest and is forced to reach a place that is suitable - as temperature and humidity - for the growth of the fruiting body, this way completing the life cycle of the fungus.

So let's invert cause and effect, and say that this fungus can infect a human host (finding a suitable place, for instance, in the digesting cable). It will then release in the blood some enzymes that alter the hormonal balance of the man, by giving him the feeling of being in love. In other words, in truth is the hanahaki that makes the man fall in love, even if to people it would seem the opposite; however the man must already have an existing feeling for somebody, otherwise the enzymes won't work.
This way the fungus has some advantages:
- the man will be forced to eat less (because of heartbreak), so that he won't disturb the growth of the fruiting body
- the heartbreak will have an impact on the immunitary system of the host, weakening it so that it can't kill the fungus.

When the fruiting body will be mature, the man will cough it, this way spreading its spores.

But luckily, if the man is able to fulfill his love, the joy will trigger another hormonal imbalance, that is lethal to the fungus, this way freeing him from the infection!

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For a scientifically correct result, there is no known way to have a 'cause and effect' relationship between an observed third-party emotional state, and the likelihood of contracting a rare disease. In addition, there are no known diseases that allow for a full-life cycle vascular plant to grow in the inhospitable conditions inside a human body.

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  • $\begingroup$ Joe, this is very true and completely worth bringing up. It should be a comment (or a couple, if you run out of space) and not an answer though, since it doesn't attempt to provide an answer to the question (even with a frame challenge). $\endgroup$ – Cyn Apr 15 at 16:58
  • $\begingroup$ The original answer had exactly what I meant, and it did provide a very clear and incontrovertible answer. I have trimmed the parts that seemed to have confused people. $\endgroup$ – Joe Apr 15 at 17:27
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    $\begingroup$ Sometimes the answer to a question is, "Sorry, it can't be done." I don't see any problem with this answer. $\endgroup$ – James Apr 15 at 17:48
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    $\begingroup$ This seems to be a misrepresentation of the question at best.. Crafting a fictional, but scientifically plausible, "emotion -> physical change -> something-which-carries-disease-detects-physical-change" chain is surely possible (mosquitos detect our carbon dioxide emoissions). Here is a story about a pea plant growing in a man's lungs. The OP talks about curses for god's sake; saying "nope, sorry" is just not an answer here. Saying the words "scientifically correct" doesn't make this answer any more credible. $\endgroup$ – AmagicalFishy Apr 15 at 20:17
  • $\begingroup$ If you're splitting hairs, OP said "Scientifically Correct" Not "Plausible", mosquitoes smelling CO2 is in no way related to disease susceptibility as a result of a third-party emotional state, and the "pea in the lungs wasn't as a result of a disease, but of unfortunate eating habits $\endgroup$ – Joe Apr 16 at 13:57

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