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.