I've been toying with a story of person sent to a alternate reality that mirrors medieval Europe, but not necessarily with identical history/countries. The time traveler would earn the trust/support of the youngest prince, but the prince's father, the king, still suspects him to be a charlatan and distrusts his 'future' knowledge.

I want the time traveler to finally be tested by being asked to help 'cure' a disease rapidly spreading in a nearby country. I've attempted a question on this topic before, but I was told my theoretical disease wasn't realistic, so now I'm asking for a realistic disease that he could help cure.

I don't want this disease to be one he happens to just remember the cure for, as I want him to have to work at it. This pretty much rules out the Black Plague, and I'd prefer to avoid creating any plague similar enough to Black Plague for readers to see it as a Black Plague knock off. I have considered smallpox, with his remembering cowpox was used as the first vaccine for it, but I'm honestly not sure how he would handle reliable infecting a population with cowpox and am also not sure if he can convince the king(s) to support the action, basically I'd prefer some other option, one where he has to use his knowledge of scientific method and how disease spreads to come up with a means of pinpointing how it spreads and prevent it.

The obvious answer is to tell everyone to use proper sanitation, but I don't want to go that route either. He has told everyone about proper sanitation, but people are resistant to the ideas. Doctors hate the idea of being told their a major spreader of disease and can't be talked into using boiled water to wash their hands (which is something that definitely happened back when sanitation was first recommended), and the king is not convinced of our Travelers reliability enough to be willing to fund expensive sewer or aqueducts on his say so. Basically he doesn't have the political clout to convince many people that sanitation is important yet, and it's hard to prove the benefits of sanitation definitively if he can't convince people to try it. Also, while sanitation is important, it's not as obvious a cure. I want the Traveler to find a cure that is undeniable to back his credibility up. Something as definitive as "these fleas spread the disease, kill all the rats and look the disease isn't spreading, there can be no doubt that this is the solution."

The Traveler is not a doctor and does not have a medical degree. He is however intelligent with a relatively wide breadth of knowledge in science. He also has an interest and more thorough depth of knowledge in evolution in particular, which has enough overlap with biology and evolution of disease to have some idea of how to handle disease.

I am therefore looking for a plausible disease that he can cure which fits a few criteria.

  • The disease is new and spreading rapidly enough to have drawn the attention, and concern, of local kings
  • He can find a 'cure' to, be that a way to minimize it's spread or treat those already infected
  • The cure is definitive and obvious enough that he can convince everyone to support it despite the king's initial skepticism of him.
  • He could plausible figure out a means to address the disease with his limited medical knowledge and scientific method, ideally without his requiring too massive an expense or logistical exercise to discover the cure (the king of the country infected is desperate enough to support our Traveler's efforts to some extent, but his king's skepticism still means that support is limited)
  • is not something he cures just by remembering how it was cured in the past
  • His cure can't just be to tell everyone to practice better sanitation.
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    $\begingroup$ Ironically, killing the rats to prevent plague might increase the problem, at least at first. Fleas prefer the rats. If there isn't a rat, they try the dogs and cats first. If they are spent, they go for humans. The rats are nearly unstoppable if there is food in the city, so killing them yourself would just increase the fleas without a rat to ride while the new flea ridden rats still come into the city. Just a side note. $\endgroup$
    – Trioxidane
    Commented Apr 22, 2021 at 14:55
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    $\begingroup$ Would a fictional disease be fine? A disease that, perhaps, is similar to real world diseases. Perhaps a disease that attacts the lungs, similar to covid-19, but in medival europe if you want to add humor. Teburculosis also existed back then, but if you want to make up a fictional disease, i'd say go for it. $\endgroup$ Commented Apr 22, 2021 at 15:21
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    $\begingroup$ Don't rule out smallpox. Once the king of Spain lost a child to it he was convinced to do a mass immunization in a very strange (to us) way. He sent a ship with orphans infected with cowpox to the colonies. daily.jstor.org/… $\endgroup$
    – Allan
    Commented Apr 22, 2021 at 22:19
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    $\begingroup$ Smallpox isn't so bad. Even if you couldn't recognized cow pox to save your life (pardon pun), variolation was a thing well before cowpox was identified. nlm.nih.gov/exhibition/smallpox/…. $\endgroup$
    – DWKraus
    Commented Apr 22, 2021 at 22:53
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    $\begingroup$ sepsis. Use alcohol on the wound to prevent $\endgroup$
    – nzaman
    Commented Apr 24, 2021 at 9:06

9 Answers 9



If you want to emphasize methodical scientific method you could do worse than to copy John Snow, one of the founders of modern epidemiology.

150th Anniversary of John Snow and the Pump Handle

John Snow, M.D. (1813--1858), a legendary figure in epidemiology, provided one of the earliest examples of using epidemiologic methods to identify risk for disease and recommend preventive action (1)... On August 31, 1854, London experienced a recurrent epidemic of cholera; Snow suspected water from the Broad Street pump as the source of disease. To test his theory, Snow reviewed death records of area residents who died from cholera and interviewed household members, documenting that most deceased persons had lived near and had drunk water from the pump. Snow presented his findings to community leaders, and the pump handle was removed on September 8, 1854. Removal of the handle prevented additional cholera deaths, supporting Snow's theory that cholera was a waterborne, contagious disease... Snow's studies and the removal of the pump handle became a model for modern epidemiology.

If you dig this sort of thing it is fine reading. Even though at the time there was still much disagreement over what actually might be in the water to cause cholera, they believed Snow's analysis enough to remove the pump handle and keep people from drinking the water and catching cholera.

  • 9
    $\begingroup$ I also thought of this exactly. At the time it was thought cholera was spread by "bad air", not by water. Snow had an alternative theory and tested it by developing a map of where the deaths had occurred. (The case is not only studied by epidemiologists but also in the data analytics and data visualization field.) The story is even structured like a modern mystery show, including interviews with witnesses (the parish priest who held the funerals and helped Snow find the pattern), and the dramatic climax (removing the pump handle). $\endgroup$
    – workerjoe
    Commented Apr 22, 2021 at 19:13
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    $\begingroup$ "I do know some things..." $\endgroup$
    – Qami
    Commented Apr 22, 2021 at 19:19
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    $\begingroup$ @workerjoe There was even an exception that he couldn't figure out at first, a lone widow far from the pump who got infected, someone who didn't fit the pattern. Then they learned she really liked the pump water from her youth and sent one of her servants out every day to collect water from that specific well. $\endgroup$
    – Nzall
    Commented Apr 23, 2021 at 13:43
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    $\begingroup$ your answer made me think and realize that any disease with diarrhea and vomiting is an easy 'cure' for our layman protagonist. The method of spread can be presumed to be diarrhea etc which in turn implies it's primarily spreading from getting into the water supply. Since he can infer all that just from hearing the symptoms he can easily tell nobles to boil their water and boom nobles are not getting sick. Then he should have enough credibility to push for sewage system. Only problem is it may be too easy... $\endgroup$
    – dsollen
    Commented Apr 23, 2021 at 14:55
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    $\begingroup$ @dsollen yes, cholera is too easy, and too well-known. Readers will either be frustrated by the character floundering at something so obvious, or this part of the story will have to provide no challenge to the character. Since you were previously open to creating a fictional disease, why not make a waterborne disease with different symptoms? Then he can go the John Snow route with interviews etc. and it will play out like a detective novel and could be an exciting challenge, and it's still grounded in the reader's knowledge that "oh yeah water spreads disease sometimes". $\endgroup$
    – minseong
    Commented Apr 24, 2021 at 15:20

Scurvy. It spreads mostly in ocean going navy, but not necessarily. The time traveler kind of suspects the disease, but does not remember the symptoms (can you?), apart from teeth falling out. He also knows the cure is vitamin C, and definitely knows that lemons (or similar) would be very good, but... there are no lemons. Nobody knows such fruits when described. The traveler might remember vegetables such as cabbage are acceptable as well, but this reality did not develop cabbage storage method for the winter (or seagoing) period. So he is up to trying to rediscover sauerkraut... (not an easy task for a contemporary city dweller).

Alternately, if he happens to know a little bit more about vitamins and food preservation, the disease could be beriberi - he might remember something about Japanese army and white rice, but... what exactly was it about the rice? (Or it might happen there is no rice. Either he is too much to the north geographically, or rice has not been cultivated in this reality). Up to experimenting, anyway.

  • 4
    $\begingroup$ "He also knows the cure is vitamin C, and definitely knows that lemons (or similar) would be very good" goes against one of the OP's requirement "is not something he cures just by remembering how it was cured in the past" $\endgroup$
    – L.Dutch
    Commented Apr 22, 2021 at 15:10
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    $\begingroup$ Also, Scurvy does not well fit the "a disease rapidly spreading in a nearby country" criteria. $\endgroup$
    – PcMan
    Commented Apr 22, 2021 at 15:20
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    $\begingroup$ @L.Dutch-ReinstateMonica The contrary: he remembers the cure, but alas, it is not applicable - so he emphatically does not cure it by remembering the cure... $\endgroup$ Commented Apr 22, 2021 at 15:54
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    $\begingroup$ @PcMan It started to rapidly spread in a British navy - once they started journeys longer than a certain threshold; and the second time, after they forgot the cure(!) and switched lemons for limes... now, "the navy of a nearby country" might not quite fit the requirement, but it is close... And it it quite conceivable that a new, rapidly spreading (and very convenient) staple food (see potatoes in Europe) replaces some major source of vitamin C. $\endgroup$ Commented Apr 22, 2021 at 15:58
  • $\begingroup$ other dietary diseases might work, look at pellagra. $\endgroup$
    – John
    Commented Apr 22, 2021 at 17:02

Any Gram Positive Bacterial Infection Works

A variation of Pneumococcal, Listeriosis, or Diphtheria infections would probably be the most likely cases of gram positive bacterial infections with the potential to be described as both an outbreak and potentially dangerous. While viruses need specialized vaccines, most bacterial infections can be treated with the same kinds of antibiotics no matter what they are; so, even without knowing what kind of bacterial infection a person is faced with, your time traveler could whip up a general antibiotic like penicillin and cure the infection.

While there is some risk of accidentally poisoning people with the wrong kind of mold due to lack of medical knowledge, the time traveler would have all the resources at his disposal to prevent this. He would just need to consult with the local cheesemaker. The type we use in medicine (Penicillium Chrysogenum) has been used in the cheese industry for over 1000 years; so, if your time traveler were trying to figure out how to safely make penicillin, he would only need to describe the green fuzzy mold that prefers growing on citrus fruits that he learned about in his high school science class, and the cheese maker would know exactly what he is talking about, and be able to sell him a safe-to-eat culture to start with. Infact, the cheese maker would already be familiar with the whole process of safely cultivating Penicillium Chrysogenum and he would know that is has medicinal properties since it was used by medieval doctors to treat open wounds. What the cheese maker would not know is that it can also be used orally to treat other kinds of ailments. So, it would just be up to the time traveler to figure out the best way to dose out an oral treatment.

The time traveler already knows you should take antibiotics for a few days after symptoms disappear, because that is common knowledge to us. Figuring out how much penicillin to use will take some trial and error, but at this point it's just a single variable of uncertainty which should be pretty easy for him to work out quickly.

  • 9
    $\begingroup$ It took scientists a strike of luck to be able to discover penicillin. Why would a person with no previous knowledge (as per OP request) hit bingo at the first try? $\endgroup$
    – L.Dutch
    Commented Apr 22, 2021 at 15:47
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    $\begingroup$ Potentially-relevant answer about difficulties in creating penicillin from scratch. worldbuilding.stackexchange.com/questions/13493/… $\endgroup$
    – notovny
    Commented Apr 22, 2021 at 18:20
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    $\begingroup$ @Alexander Native Americans used Penicillin mold as is to treat bacterial infections long before Fleming came along. You don't need a cure that will not kill you 100% of the time to stop a plague. By medeville medical standards, if it saves you more often than it kills you, it's a cure. $\endgroup$
    – Nosajimiki
    Commented Apr 22, 2021 at 19:41
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    $\begingroup$ @notovny I am aware of that post, I just don't agree with it. Not only did the Native Americans use Penicillium specifically, but many Ancient cultures used various kinds of mold to treat bacterial infections. history.stackexchange.com/questions/56431/… $\endgroup$
    – Nosajimiki
    Commented Apr 22, 2021 at 20:02
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    $\begingroup$ Also, the antibiotic properties of these cheeses mold was already well known in medieval times for wound care. Cheese mold was often packed into wounds to prevent infection which we now know worked because of the Penicillium. The protagonist just needs to apply what he knows about the future to know it's good for more than just open wounds. $\endgroup$
    – Nosajimiki
    Commented Apr 22, 2021 at 20:50

I’m going to slightly challenge your premise here by arguing that:

The exact disease does not matter much given your constraints.

Put simply, if you want an epidemiological solution (and you seem to be asking for that), all that matters is that the means of transmission of the disease be readily discoverable by someone with a layman’s understanding of germ theory and that the minimum exposure required for infection is not ridiculously high (not having a super high contagiousness will help make it more believable that the protagonist does not become infected while researching how the disease spreads).

With some very limited cases of airborne pathogens, countermeasures to limit exposure are both very simple and will quickly show results (well, quickly relative to the incubation period of the disease). It doesn’t have to be complicated, basic stuff like avoiding the body fluids of infected patients (Ebola, most types of viral hepatitis, most venereal diseases, etc), or proper food storage (ergotism, salmonellosis, etc), or ensuring clean drinking water (cholera, amoebiasis, etc), or even just not touching your eyes (viral or bacterial conjunctivitis) will work here. The important thing is simply that the protagonist can figure out how the disease is spreading, and implement measures to deal with it.

The only caveat to this is that I would strongly suggest avoiding diseases that have insects as a primary transmission vector (malaria, plague (the actual plague, caused by Y. pestis), Lyme disease, yellow fever, etc) as they are hard to track properly unless you have reason to believe an insect is responsible, and countermeasures are difficult at best without modern technology (you can reduce risk somewhat, but much less so than for something like cholera or ergotism). Myiasis is a possible exception (once you hear the symptoms, it’s immediately obvious what the cause is), but that is likely a bit too grotesque for your tastes.

  • $\begingroup$ per the answer with cholera I realized that any disease with vomiting's & diarrhea is pretty easy for a layman to 'cure'. The method of spreading is obvious with them (the diarrhea and vomiting) which strongly implies it's spreading through the water supply. From there basic sanitation steps work. Tell the nobles to boil their water, suddenly nobles aren't getting sick. From there he likely has gained enough credibility to convince people to at least try a sewer system. Only problem I have is it may be too easy for our protagonist... $\endgroup$
    – dsollen
    Commented Apr 23, 2021 at 14:24
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    $\begingroup$ @dsollen If you want it to be challenging, then that limits things a lot more. Perhaps a zoonotic disease would be a reasonable choice then? That would probably still need some generic countermeasure to prevent spread among humans, but would still require tracking down the carrier species to completely eliminate the threat of the disease. Various hemorrhagic fevers (Ebola, Dengue, etc) might be a decent candidate in this case, as could anthrax, cryptococcosis, Q fever, orf, or even tuberculosis or smallpox. $\endgroup$ Commented Apr 23, 2021 at 14:44
  • $\begingroup$ Insect vectors can be quite simple to combat once identified, and a future traveller with knowledge of symptoms will have a big advantage in already knowing an insect is responsible. My suggestion was going to be Lymes disease, since any modern traveller with knowledge of the outdoors can look around and go ‘You all have flu-like symptoms. You all have tick bites. You’re all walking through long grasses with no protection. Wear shoes and trousers’ and boom: problem solved. It’s one of those ‘obvious once you already know the solution’ problems. Obviously this depends on the insect... $\endgroup$
    – Joe Bloggs
    Commented Apr 25, 2021 at 6:02
  • $\begingroup$ @JoeBloggs The question specifically asks for it to not be solved by just recognizing the disease, and if we accept that requirement, insect vectors become somewhat more complicated to deal with because they’re difficult to identify in the first place without somewhat modern technology. Recognizing them as a possibility does give a significant advantage, but without recognizing the disease, you have no reason to expect the transmission vector to be insects just based on simple statistics (most diseases do not involve an insect as a transmission vector). $\endgroup$ Commented Apr 25, 2021 at 12:37
  • $\begingroup$ @AustinHemmelgarn True, but even knowing to look for insect bites (and recognising that certain insects exist in certain environments) is a very obvious epidemiological marker if you know to look for it (again, for certain insects). If the disease features excessive flatulence followed by spontaneous lethal nosebleeds but there's an inflamed (like Lymes disease) bite site and something in the environment like long grasses, excessive spider webs, stagnant water etc then the time traveller can easily work out that 2+2 = insect vector and also easily suggest an effective counter. $\endgroup$
    – Joe Bloggs
    Commented Apr 25, 2021 at 14:14

Parasites are the way to go.

Most bacterium can't be cured without antibiotics, or other microscopic creatures or help. The best bet would be viruses and/or parasites. Malaria might be a good choice if it was set closer to the equator, in a hotter, more humid area. The major signs of a disease are fevers, chills, aches, and the like. Often times coughing. What if you creat a disease similar to rabies, transmitted through animal's saliva. The cure is obious, as all the paitents have been bit. If you don't want an animal transmitted disease, then perchance you can do something similar to, as I said earlier, Malaria. This will make it more of a challenge to find out the source, as the bite mark is small. A malaria/rabies like plague would be your best bet. Here is my idea:

Cities in the middle ages were very unsanitary. People have been noticing the birds, specifically crows/ravens have been acting up. People were being attacked. People took this as a sign from whatever god(s) exist in your world, and start trying to repent. Then people start falling ill. They gain a very high fever, coughing up blood, the whole shabam. The birds are spreading a disease.

If that is too black death rip off, then perhaps it was caused by worm eggs in a new spice that is being imported from some other country. It is a new spice, and people are loving it. It tastes unlike anything they have tasted before. Better yet, it is insanley cheap for a spice back in the day, and more upper-middle class citizens have access to it. Similar side effects, the main one being coughing up blood. This is how the eggs spread, through bodily functions. Many upper class people are being affected by the spice, and the people living in the unsanitary areas are being affected by the diseased grossness that many people lived in. The solution would be to ban the spice, and quarentine/kill the sick as well as everyone else. This will stop the spread as much, and it will eventualy kill it off, maybe if you make the eggs die if exposed to sunlight for too long.


That disease caused by ergot fungus rye contamination. Just have him point out the bread is making their toes drop off and making them act nuts .

  • $\begingroup$ I think this would be a fine choice. Traveller could sense something amiss with the locale's food stores, get some thugs to burn them all and the sympathetic prince to supply replacements. Also good: milkborne diseases. Traveller can retro-invent Pasteurizaton. $\endgroup$
    – CCTO
    Commented Apr 23, 2021 at 15:53


Measles are highly spreading infections of the skin.

The time traveller could find that Vitamin A helps preventing blindness due to the infection (which could be a partial proof of his skills to the king) but ultimately only quarantine and washing with soap would help preventing the disease. Also, the practise of infecting young children to make them immune could be a common knowledge in our time but not back then. Also, general use of soap plus drinking huge amounts of water to prevent dehydration due to high body temperature could reduce mortality.

Ultimately, your Traveller would need to find a (fantasy) cure for it, like a special herb that purges the skin from viruses or something, because he could for now only fight the symptoms.

If you don't like the idea of viruses that can't be stopped, you could go for a fungus infection of the skin. Then the traveller could find out that mere camomile tea works antiseptic, thus killing fungi. As a side note you can even cure fungus infected plants in your house with camomile tea due to these traits of the camomile. Maybe the fungus is not only on their skin but also in their stomach and suppresses the body from gaining nutrition.

If this all isn't right for you, go with vampirism lol

  • $\begingroup$ good idea. Though some quick googling implies soap was a very rare commodity back then, so not sure it would prove a cost effective viable strategy for most of the population. I'm totally okay with his knowing he could use measles to vaccinate against measles, if I know how it was done it's reasonable he would. but convincing people to risk a non-trivial chance of death to lower their odds of death if they were exposed to measles, even if logical, is a rather hard sell, I'm not sure he has the clout to make that sell. $\endgroup$
    – dsollen
    Commented Apr 23, 2021 at 13:16
  • $\begingroup$ true, that's why i said maybe a funghal skin infection? there are cases of people having funghi infection on their genitals, maybe in your world the infection could spread to peoples mucous membrane in the stomach or the mouth because they eat mushroom infected food, like salad or wheat. the obvious "cure" would be first to decontamine the ground where they grow their food ( or the fields where they hold their cows ) and also to find something that kills the mushroom, like the camomile tea, which would make sense a common human could find out. interesting question! $\endgroup$
    – user59660
    Commented Apr 23, 2021 at 14:33
  • $\begingroup$ also, the measles would turn ppl. red, so other folks might see them as "demons", giving the whole setting a rather dark souls like touch - and since it is explicitly stated kind of a "parallel world" - well, maybe there IS a cure for this besides infecting yourself - that would be the author's job. i don't know how sci-fi this setting should go, but maybe there is a 3rd party from overseas / far away who have a cure and nobody listenes to them since they are outsiders $\endgroup$
    – user59660
    Commented Apr 23, 2021 at 14:41


You say he's not a doctor but he has a wide breadth of scientific knowledge, so he might just know how to invent penicillin. To be honest I don't know how easy it would be to invent but Professor Arturo does it in Sliders and he's not a medical scientist...



Stuttering is a speech impediment that is thought to be genetically inherited. If one were to take liberties of saying its a common but recessive gene, the gene could be passed through families like a sleeper cell, and then become massively widespread and dominant all at once under certain conditions, producing a generational snowball effect of stutterers. Though not a disease, but a disability, stuttering could become endemic to a community or culture.

You mentioned your character is more interested in evolutionary biology, and so they would be more attuned to a genetic transmission medium such as this, and as a result, might have a better understanding about how to begin tackling such a problem.

Here's an example how. Not through genetics (they are no scientist or doctor after all), but through modification of the affected brain. This could be accidental at first - blunt force trauma to a particular area of the head, or being impaled, or being in a coma such that part of the brain dies and the patient must relearn to speak with another part of their brain.

From there, other stepping stones, experiments, observations could be made to give this character a decent understanding of where the malformation occurs, and subsequently the ability to 'cure' it in affected individuals through various or particular means.

This reminds me a lot of the main character in the movie The Physician, who went from knowing nothing about medicine or surgery, and by the end of the movie

successfully performs an appendectomy on a King


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