One of the most well known, but commonly ignored issues of time travel is this:

Due to the fact that viruses and bacteria constantly evolve, diseases that we are today resistant to have never before been seen in history. This means that if we were to travel back in time, there would be a large chance we could infect someone and wreak havoc on the world.

Answers looking at a timescale for an event like this would tremendously help people writing stories about time travel looking to be as realistic as possible.

With the current situation of the Corona virus (as horrible as it is), we have a fairly clear answer as to what would happen if a time traveller from the future infected us today.

However the question "what happens to the world if?" is much too broad for this site, so I will try to severely downscale this in the hope of making this question answerable.

Location: London City.
Time: 1900 AD.
Spread: 1 market stall owner.
Disease: Covid-19 anno 2021 (when all of this is hopefully over).
Time travel paradoxes: Alternate time line created at point of travel.

For the sake of this question I would like to know what would happen mostly medically in just London from day 1 to day x, where day x is either the entire population wiped out, or the virus is no threat anymore (virus eradicated or survivors have built up reasonable resistance).

Any answers and predictions welcome. Please let me know if this question is still too broad and what would it would need to make it answerable.

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    $\begingroup$ Bad choice of virus -- seasonal flu is always new, so the impact of the 2020 seasonal flu on 1900 London will be just about the same as the impact on 2020 London. (This is why people who are very concerned about the flu have to take annual vaccinations. The flu circles the world; so each year vaccines are prepared from the strain in Shanghai and Hong Kong and distributed worldwide, providing a degree of immunity to that specific year's seasonal flu. That's also why having the flu in one year won't confer immunity for next year's flu.) $\endgroup$ – AlexP Mar 13 '20 at 10:28
  • $\begingroup$ @AlexP good point... Its just the most common one to spring to mind. Any suggestions for something that did not exist then? $\endgroup$ – Plutian Mar 13 '20 at 10:30
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    $\begingroup$ I can't think of anything specific. If anything, I would expect the time travellers to fall prey to any number of bacterial or viral infections, unless they had taken the precaution to get vaccines as if they were going to an underdeveloped tropical country... How is your immunity to cholera, dysentery, typhus, or diphtheria? $\endgroup$ – AlexP Mar 13 '20 at 10:36
  • $\begingroup$ @AlexP that would be a good answer. Changed the virus to hopefully something better. $\endgroup$ – Plutian Mar 13 '20 at 10:43
  • $\begingroup$ It might depend on whether you are talking about time travel in one timeline or subscribe to the multiple timeline theory. If it is the single timeline theory then nothing will happen because it didn't happen in our past. $\endgroup$ – mwarren Mar 13 '20 at 11:29

The diseases that we take such efforts over now are inconsequential compared to the death rates from diseases in the past. Untreated, bubonic plague kills upwards of 50%, pneumonic plague kills at 90-95%, septicemic plague kills at 100%. Even an infected cut could kill you due to lack of antibiotics, a rat bite would have a good chance of being lethal.

The primary reason for the oft quoted low average lifespan is the modal age of death was 0. Merely surviving childhood was a major success.

During the late 1800s and early 1900s, pneumonia was the leading cause of death due to infectious disease and the third leading cause of death overall. [...] A novel technique called antiserum therapy soon began, and by 1913, antipneumococcal serum therapy, if given early in disease progression, was able to reduce mortality from 25% to 7.5%. However, this treatment method was slow, costly, and time-consuming. - source

Covid-19 kills at around 1-3% (by pneumonia). It'd probably kill significantly below the modern rate as the primary affected demographics are unlikely to have survived to that age or with those underlying conditions.

In the grand scheme of historic diseases, it's background noise.


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