I have traveled back to the 15th century with the goal of facilitating the discovery of the Americas by the rest of the world without causing epidemics to wipe out 90% of the American population.

How can I achieve my goal?

The best solutions are those that can be achieved using 15th century technology and modern knowledge, but I can bring items with me if they are necessary. Further time travel should be used only as a last resort.

I do not necessarily have to enact my plan from Europe.

EDIT: I'm not sure why people are commenting on how immense the impact of this change would be. I'm not asking whether or not my plan is a good idea - I'm asking how I might achieve it.

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    $\begingroup$ The @Thorne answer is spot on. This would seriously change history, and you can't know whether the consequences of this change wouldn't have other just as drastic -- or possibly worse -- consequences. $\endgroup$ – RonJohn Aug 3 '18 at 6:33
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    $\begingroup$ @RonJohn: Of course it would seriously change history! That's sort of the point. I'm not asking whether or not this is a good idea - just how it might be done. $\endgroup$ – Arcanist Lupus Aug 3 '18 at 6:36
  • $\begingroup$ OK, just checking. (Questioners don't always think things through.) $\endgroup$ – RonJohn Aug 3 '18 at 6:57
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    $\begingroup$ You might have already thought of that, but it's worth mentioning explicitly that you and your time machine better be decontaminated thoroughly for each travel, otherwise you risk destroying your efforts by introducing future diseases in the past, and vice versa. $\endgroup$ – AmiralPatate Aug 3 '18 at 9:53
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    $\begingroup$ I don't know, I think attempting to save America from European plagues in the past and accidentally reintroducing smallpox and bubonic plague to the modern age is a valid story line. Time travel stories are all about trying to undo the damage you did with time travel in the first place after all. $\endgroup$ – Separatrix Aug 3 '18 at 10:13

13 Answers 13


Pre-emption is the best prevention

There is no way to do it with 15th century technology. But.

The best course of action is to pre-empt the epidemics by running them early. Use your time traveling saucer to kidnap some flea-infested bug-ridden mangy European and Chinese serfs in the 5th century, and drop them in the Americas. The epidemics will start, lots of native Americans will die, but some will survive; and those who will survive will grow and multiply, while living in equilibrium with the diseases. By the time the Europeans will get around to discovering the Americas in the 15th century the New World will be teeming with multitudes of disease-resistant Indians.

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    $\begingroup$ (+1) That's the only possible solution, since you can't vaccine the whole american native population. $\endgroup$ – Rekesoft Aug 3 '18 at 10:22
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    $\begingroup$ @Rekesoft - One could look at it as vaccination via natural selection. $\endgroup$ – T.E.D. Aug 3 '18 at 13:12
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    $\begingroup$ And then when the navigators come, they get a nasty disease that wipes them out. $\endgroup$ – Mindwin Aug 3 '18 at 13:29
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    $\begingroup$ @Mindwin - They kinda did: syphilis. It killed a pretty decent amount of Europeans in the mid 1490's. $\endgroup$ – T.E.D. Aug 3 '18 at 14:26
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    $\begingroup$ @t.e.d. oh yes, the "swine lover"'s affliction. $\endgroup$ – Mindwin Aug 3 '18 at 14:29

Possibly, if you make two trips and don’t mind mad science and teaching. I think you need to make two trips regardless, but it’s possible you might not.

The first trip is to gather samples of all the communicable diseases you want to fix from the Europeans. This is important, because diseases mutate and change at a terrifying rate, so a viable vaccine for today’s influenza is not a viable vaccine for the influenza of yesteryear. You then need to bring them back to today for step two, which is:

Make a communicable vaccine. Mad science time, and the part of this most likely to go horribly wrong. As have been noted in a couple of other answers (Mass disease prevention: The feasibility of airborne vaccines and Vaccines spreading as viruses) there exist live-attenuated vaccines which can vaccinate and also spread. There are a couple of downsides but I’ll come to them. If you can create your own custom (heavily gm) version of this you can have a highly infectious but remarkably asymptomatic vaccine, in much the same way that cowpox, while infectious, vaccinated against smallpox. Given that nobody will have immunity to the virus it will spread like wildfire, not killing anyone but making sure everyone is safe when the actual killers show up.

Now, this can go wrong in a couple of ways. The first is the most obvious: your vaccine is a little too much and kills everyone anyway before their immune systems can kick in. Your vaccine needs to walk a super-fine line between being infectious enough to spread and being weak enough to not overtax the host, while also kicking off all the appropriate immune responses and creating the proper antibodies. This is not a small task. You may wish to bring some blood samples and/or test subjects from the past to the future to.. erm... make sure you get it right...

The second is less obvious and oddly, either much less deadly or absolutely deadly. Your vaccine will have every opportunity to mutate back into a lethal version of the disease. Now, in the best case this will kill a tiny fraction of people. Everyone else will have already caught and fought off the vaccine disease. In the worst case scenario the new disease mutates in such a way that the old vaccine is useless, and you just introduced the killer epidemic.

But this is where part three comes in! Education. Basically if your second trip to the past goes back a long enough way you can make sure the vaccine diseases have spread and also set up a known network of hospitals/health education centres. Get the mayans onboard with medicine. Teach the Incas about cleanliness and covering your mouth when you sneeze. Convince the priesthood that people who feel ill need quarantining for a little while, and get some idea of epidemic preparedness going on. A little knowledge can go a long way in situations like this. If even one chief realises he should stop ten ill travellers from travelling it might stop a potential epidemic (literally) dead.

Hopefully you’ll get lucky and be able to stop this, but all these steps are sort of longshots, some with the possibility of accidentally unleashing an even worse plague.

So please change the past responsibly.

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    $\begingroup$ Vaccines already kill a small percentage of those who get it. $\endgroup$ – RonJohn Aug 3 '18 at 7:47
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    $\begingroup$ @RonJohn: that’s generally through a reaction to the various chemicals present in the vaccine, not the vaccine ‘reactivating’. A killed vaccine basically only contains the bits of the disease responsible for stimulating an immune response but none of the parts required for replication or doing damage while a live vaccine contains a crippled but still viable version of the disease. In populations with relatively good immunity the former is preferred because it reduces the chance of the disease itself killing someone to practically nil. $\endgroup$ – Joe Bloggs Aug 3 '18 at 8:02
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    $\begingroup$ @RonJohn: that’s the whole point of my answer: a GMO that spreads through the population without needing syringes is basically the only way. Hell, I could infect myself and walk from village to village sneezing as an effective delivery method. $\endgroup$ – Joe Bloggs Aug 3 '18 at 8:27
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    $\begingroup$ Spreading this GMO vaccine throughout two very large continents covered in virgin "old growth" forest, raging rivers, steep mountain ranges, swamps, etc, etc is... more than a daunting task. $\endgroup$ – RonJohn Aug 3 '18 at 8:32
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    $\begingroup$ @RonJohn That's misleading enough to be almost untrue. "A study published in 2013 using electronic health record databases reviewed health information on over 13 million vaccinated persons and compared causes of death in the vaccinated study population to the general US population. The death rate 1 or 2 months following vaccination was lower than that in the general US population, and the causes of death were similar. This study provides convincing evidence that vaccinations are not associated with an increased risk of death at the population level." $\endgroup$ – Tim B Aug 3 '18 at 16:05

Here's a radically different tactic (that also messes with the timeline a lot more) for you to try.

The reason the Europeans carried so many diseases had little to do with their geographical location and more with the way they lived. Big, cramped cities, thousands of people living close by, inadequate waste disposal and the occasional open sewer were the perfect breeding grounds for a metric crapton of diseases. Add pests like rats and pigeons and you have a pandemic waiting to happen.

So give the peoples of the Americas the exact same odds. Travel to their neck of the woods a good bit of time earlier (100-200 years should be fine) and introduce them to the wonders of urbanisation. Point out sites of natural resources and show them how to exploit them. The natural result of a mini-industrial revolution will drive them to build cities where they'll live close to each other and start breeding their own diseases. Perhaps introduce rats or a similar pest and let the incubator start.

To make sure there's no horrible double-extinction, maybe travel over to europe every so often (bring a future-tech modern boat, no-one has time to sit on the ocean for months on end) and exchange some of the pests and other sources of disease, making sure the two continents remain on par in terms of death by microbial life. When one of the two continents inevitably sails over to the other, they will both be at the very least resistant to the diseases that they bring.

And as a bonus: maybe the Europeans don't wipe out most of the native culture.

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    $\begingroup$ Interesting idea, but 200 years is not enough. You'd need to bring domesticated draft animals like horses and cattle, plus showing them where to dig for iron and copper, and how to smelt it, etc. Otherwise, there won't be the surpluses needed for urban life. $\endgroup$ – RonJohn Aug 3 '18 at 13:55
  • $\begingroup$ @RonJohn If I'm not mistaken, by the time the europeans showed up, some amount of domestication had already taken place in the americas. I'm thinking specifically of things like corn, llamas, dogs and a few other plants and animals. Making the leap shouldn't be too hard if you provide them with the correct incentive (wealth and power) $\endgroup$ – Valthek Aug 3 '18 at 14:01
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    $\begingroup$ Llamas are the only large domesticatable pre-Columbian animal in the Western Hemisphere, and they're not very big. Nor can they haul plows, nor are they anywhere but the Andes. $\endgroup$ – RonJohn Aug 3 '18 at 14:07
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    $\begingroup$ The big thing they lacked was a domesticated draft animal, that means no plows and limited long distance travel/trade. Those are the big factors you need for surplus. on the upside if you introduce them humans will spread them for you. $\endgroup$ – John Aug 3 '18 at 15:13
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    $\begingroup$ having a draft animal is what gets you the surplus to get the other things. They had wheels but they are basically useless without something to pull it. Long distance travel was actually fairly common in civilizations with draft animals, there were major trade routes through three continents. heck oxen, wheat, and horses got to most of eurasia and africa through early trade routes. food surplus are what produce advanced technology. $\endgroup$ – John Aug 4 '18 at 3:12

Nothing, even if you were to pre-invent vaccination, distribution will only spread the diseases if attempted, because you will need to use lots of people to distribute it. And that's assuming you somehow get pass the cultural barrier and get millions of natives to agree to let you stick needles into and inject things into their wives and children.

the only thing you could do is introduce the diseases earlier, like hundreds of years earlier, and let the native repopulate before the europeans come, and hope enough of them retain a genetic resistance. Of course this will make it even easier for the europeans since the natives will now be in small easily overcome groups following the destruction of all the large population centers. Worse if you introduce ALL the relevant diseases at once you may very well wipe out humans on the continent.

  • $\begingroup$ "the only thing you could do is introduce the diseases earlier " What about 2000 Years earlier? Then the deseases will be as normal as they are in europe at the same time. Of cause you cannot be 100% safe because deseases change over time, but it will help. Of cause you must know that you change the History of the whole mankind. If the maya and aztec are not killed by the diseases the europeans bring with them, it will not possible to conquer america and the whole history changes. $\endgroup$ – Julian Egner Aug 3 '18 at 6:34
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    $\begingroup$ it will not possible to conquer america It would definitely increase the challenge rating but I don't know about impossible. Europeans would still have guns and a sense they were entitled to the world's riches after all. $\endgroup$ – AmiralPatate Aug 3 '18 at 10:02
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    $\begingroup$ @JulianEgner, the problem is the diseases need large population centers to stay in circulation, the earlier you introduce them the smaller the proportion of the population that will be resistant in the targeted period. 2000 years earlier many of the cultures affected didn't even exist, so you also have the issue that they will not spread effectively. $\endgroup$ – John Aug 3 '18 at 15:08
  • $\begingroup$ +1 for logistic freaking nightmare. Plus another one for this is an IPS question - I hope you put all your points into charisma... $\endgroup$ – Mazura Aug 4 '18 at 0:01
  • $\begingroup$ I was answering to a comment and I forgot to tag the commenter @JulianEgner $\endgroup$ – AmiralPatate Aug 6 '18 at 5:45

On request by Joe Joe Bloggs:

This question is actually a lot harder as it appears on first glance. If you innoculate the population of the Americas you merely put back the disaster one generation (or less depending on how long the immunity from inoculation is actually for). So, you have to be willing to keep it up in the new world forever (they'll be no push to bring the live vaccines over if the diseases mysteriously aren't appearing) or fix it another way.

I have considered two ways. You could in theory force the disease breakouts centuries earlier with the same catastrophic kill rates, but this is ethically unsound and significantly more dangerous due to increasing the genetic bottleneck that made the natives uniform enough to have one blood type.

The other solution is to kill off the deadly diseases in time to prevent the disaster. You will need multiple trips.

  1. Pick a year. 1492 will do. In fact this is a very good time as the Portuguese have started sailing around Africa.
  2. Gather samples of smallpox, measles, mumps, and greatpox (the old name for syphilis).
  3. Extract the genes for their surface proteins (DNA or RNA)
  4. Transplant the genes into a modern copy of the common cold along with activation sequences.
  5. Test your new viruses carefully. There's a small chance you made an airborne killer.
  6. Release these viruses at multiple points in the old world all at once. The greatpox one is to be used in the new world.
  7. Repeat as necessary near any residual breakouts. You have time travel that permits editing the past: you can watch the news and see where any breakouts come from and try again to stop them.

If you did your job right you have modified the common cold into an airborne vaccine or set of vaccines against the killing diseases. The genes you spliced into the cold virus aren't doing it any good so they'll be withered away sooner or later but this doesn't have to last all that long. Just long enough.

Presto. Epidemics of the Colombian Exchange halted. If anybody learns you did it they will proclaim you a hero.

So, if you're wondering how this works: The transplanted surface protein won't do much in the situation. It's either going to be incorporated into the cold's surface protein matrix to be picked up by the B-cells for antibody manufacture (we want this) or made but not picked up and eventually dumped into the intracellular matrix (almost as good). So long as we weren't silly enough to use the site-latching surface protein from the target virus that would be picked up by the common cold virus the resulting virus is nearly as safe as a cold (small chance of autoimmune disease from mumps).

  • $\begingroup$ How much of the science in creating these viruses is practical and how much is theoretical? And how theoretical? Is this "nobody has bothered to fund the study yet," or "I think you should be more explicit in step two" theoretical? $\endgroup$ – Arcanist Lupus Aug 5 '18 at 4:35
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    $\begingroup$ @Arcanist: It's we can do it tomorrow level theoretical. The same process involved in making CRISPR modifications can do this. $\endgroup$ – Joshua Aug 5 '18 at 15:04
  • $\begingroup$ From experience, even colds are population density dependent. My present life I'm on a farm, and I see a couple hundred customers a year, mostly in an outdoor setting. I haven't had a cold in years. I think you want your disease vectored by fleas or ticks, or be present in mouse droppings. $\endgroup$ – Sherwood Botsford Aug 8 '18 at 2:52
  • $\begingroup$ @SherwoodBotsford: Thankfully, of the diseases we want to kill, only syphilis doesn't die off if it can't spread rapidly, so it's not as much of an issue as expected. $\endgroup$ – Joshua Aug 8 '18 at 15:06

Considering that smallpox was the main reason for incredibly high mortality rates among Native Americans after the contact with Europeans, I would suggest to look specifically into it.

Quarantine and Isolation

These measures minimise the spread of smallpox and protect the aboriginal population.

Smallpox has the incubation period of 10-12 days. It is believed that there are no asymptomatic carriers. Moreover, people who survived smallpox are completely immune. Therefore, with modern medical knowledge, it is easy to establish 21-day quarantines for all new arrivals. Isolation of infected individuals and employment of already immune persons to care for them will prevent the spread of the disease.


Smallpox was known in Africa, Europe, and Asia from Antiquity. There is some evidence that inoculation was practised in the 16th century in China. It is a bit later than your time period, but we can assume that some ideas were already in the air prior to written mentions that we have.

At that time, either dried scabs or puss were used for inoculations. Both result in a rather mild form of smallpox with mortality rates at about 0.5-2%, which are significantly lower typical 20-30% rates for regular smallpox.

You may want to repeat inoculations every 10-15 years as the immunity tends to wear off.

Prohibition of Biological Warfare

Smallpox was used as a biological weapon by European colonists. If there were some law or religious norm that would not allow this the spread of disease would be slower and easier to control.

Implementation of these measures does not require additional time travel or any anachronistic equipment. Everything can be done using the 15th-century technologies.


Education and Sanitation.

You can't necessarily prevent diseases - Even the answers posted here assume that you can 'backport' immunity to pathogens through the timeline and expect that those immunities/resistances will continue to be communicated to all of their descendants, which is probable but not bulletproof.

But what you can do, is teach the populace how diseases spread, and how they can protect themselves against outbreaks, and to whatever means available treat infections and manage humane quarantines.

This strategy would still involve a great deal of death, but, it would definitely prevent a large portion of it, and put the indigenous peoples in a much better position once colonization actually began. As an added bonus, it doesn't do much to disrupt the existing culture or social structure prior to the arrival of Europeans.

Such a method would likely require more than one protagonist, and it would be the life's work of many people to indoctrinate such a wide number of different communities and cultures throughout two continents.

  • $\begingroup$ I love this idea. A widespread understanding of how diseases spread could prevent the mass casualties that our timeline saw. A single determined and effective traveler could possibly make it happen. I disagree with it being a minor disruption. It would be huge if Europeans showed up and found Native Americans with a developed and experimentally verifiable theory of pathogen based disease. It shifts the whole power dynamic. $\endgroup$ – jorfus Aug 3 '18 at 20:07
  • $\begingroup$ There was a GURPS sourcebook for some fantasy world that used this idea. Basically, the populace believed in something called "demonets" that worked similar to how germs really do. Convince the native healers and medicine men to wash their hands, tools, and the wounds of the injured with hot water, and they'll eventually see the survival rate will go up. People succumbed to the Black Death partly because they thought it was either airborne or the Wrath of God. Convince the natives about separating the sick from the healthy; blame it on evil spirits jumping between them if you have to $\endgroup$ – Shawn V. Wilson Aug 3 '18 at 23:38
  • $\begingroup$ @jorfus - I meant this as : "Teaching people to wash their hands will not require fundamentally altering their belief system." and not as "Disease control shouldn't have much of an effect on the timeline" I've edited the answer to make this more clear. $\endgroup$ – Iron Gremlin Aug 8 '18 at 0:59

Introduce dairy cows in North America shortly before first contact. Make sure the cattle are infected with cowpox. About one third of the population will contract cowpox, and acquire immunity to smallpox. This won't prevent the smallpox epidemic, but it will drop the death rate from 90% to 67%. That's still a disaster, but it's recoverable. Can't help with other diseases.


I would give a shoot at

Vaccinating European sailors to the new world and desinfecting their ships


Use your future technology to develop oral vaccines for the diseases you want to prevent and administer them to everyone onboarding the ship as a few drops of blessed water given by a priest.

The day before the ships sails, enter it and spread some 21st century-tech poison that kills rats, fleas or any other pest that is known to spread those diseases

A few reasons why that would help:

  1. I assume there would not be many sailors (a few dozens per ship?) and not many ports to sail from in Europe, so the control could be done by a one or a few persons in each port.
  2. Starting with a clean (no rats or fleas) ship and a crew that developed vaccine-induced immunity against the diseases during the weeks long sail, I suppose the microbes which transmit the diseases and that might have survived the decontamination wouldn't endure the trip, so there would be no disease microbes by the time the ship arrives in the brave new world.

Comparisson to other possible ideas:

The idea of vaccinating the new world population is much tougher, since you don't know where do all those indians/mayans, etc live, so very hard to vaccine most of them.

Spreading the disease a few centuries beforehand could have unexpected and unforeseen consequences, even extinction of the population, so it's hard to tell if this would work.

Finally, the idea I outlined might not work also, is hard to tell for sure, but at least it seems doable, so worth giving it a try!

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    $\begingroup$ Nice idea, but the introduction of smallpox was intentional. Cortez wasn't very nice. $\endgroup$ – Joshua Aug 3 '18 at 18:04
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    $\begingroup$ @Joshua Did not know that! Perhaps another way to prevent the spreading of some diseases would be to travel back in time and prevent Cortez to set sail! $\endgroup$ – gmauch Aug 3 '18 at 20:38

Why were Europeans less threatened by smallpox than Native Americans? One reason was cultural:

We concluded that transmission was controlled in southern England by local practices of avoidance and mass inoculation that arose in the seventeenth and eighteenth centuries. Avoidance measures included isolation of victims in pest houses and private homes, as well as cancellation of markets and other public gatherings, and pre-dated the widespread use of inoculation.

--The geography of smallpox in England before vaccination: A conundrum resolved

Potentially the principles of quarantine could be taught to Native Americans, enough to slow the spread of smallpox. That might help with diseases like measles as well, though that's so much more contagious than anything that it's difficult to quarantine against.

Cowpox and vaccinia could in theory be transferred to North America, except they prefer cattle, which weren't in North America. Both viruses, especially vaccinia, are reasonably forgiving of precise hosts, so possible one or the other could infect bison; but I can't find any papers demonstrating that. Possibly introducing cowpox and vaccinia-infected cattle into North America could help, especially if you were to teach how to propagate the viruses from one host to the next and use the scabs as vaccine sources, as was occasionally done during the smallpox eradication campaigns.

(Cattle are also infectable with rinderpeste, which has potential as a cross-reactive vaccine against measles, but rinderpeste is so epidemic itself that it's not a good solution. Similarly canine distemper has potential as a cross-reactive measles vaccine but comes with its own multitude of problems.)


A lot of the diseases of the old world were cohosted by domestic animals. Bring domestic sheep, goats, chickens, pigs, horses, camels and cows over in around 1000 AD. Introduce them to cultures that were already starting to build permanent settlements for the smaller livestock. (Missisippi mount builders, Iroquois Confederacy, Introduce horses to prairie nomads, camels where it's drier.

See Jared Diamond's book "Guns, Germs and Steel"


You can't. The old world populations were resistant, the new world populations weren't. They're going to run like wildfire once they get loose and there's no hope of forever keeping them from getting loose.

AlexP's solution simply moves the catastrophe back in time, it doesn't change the basic result.

However, the catastrophe can be lessened. He's got the right idea but is going about it wrong. Gather samples of all the diseases in question and infect the new world one disease at a time. Each will be a major plague but will not decimate communities nearly as badly as all of them together did. You'll probably kill as many people but it won't tear up the societies nearly as badly.


I can't see how you can short of curing these diseases back in Europe or vaccinating the native population.

Even if you screened the whole crew, once a new land was discovered, other ships would come and bring the diseases with them instead.

It's really unavoidable. The real question is should you? Should your first trip in time involve you changing time? How many people will never be born from your change?

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    $\begingroup$ cure isn't even an option many of those diseases are still around. $\endgroup$ – John Aug 3 '18 at 6:17
  • $\begingroup$ Using 15th century technology, you'd inoculate the American populace. $\endgroup$ – RonJohn Aug 3 '18 at 6:55

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