In the sixth century, an outbreak of killed forty percent of constantinople's population.
ebola
sars
cholara
bubonic plaque

Bubonic plaque

Explanation:

The Bulbonic Plague was one of the greatest epidemic outbreaks in human history. The disease was contracted from human contact with fleas infected by the Yersinia pestis bacterium that were present in rats.

This disease was carried by Genoese ships to various ports on the European continent. The Genoese colony of Caffa, located in Crimea, was surrounded by the Tatars, who used the infected corpses as a weapon. The corpses were thrown over the walls into the city, which caused the disease to spread.

With the city surrounded and with the outbreak epidemic, many Genoese fled on their ships and eventually took the disease to other cities in Europe, such as Constantinople which had about 40% of the population decimated by the plague.

genetics is the study of how dna works, the methodologies defining the new recent technologies allow scientist to have a panoramic view of the entire dna landscape, especially how genomes interact with their environment, including off course, factors outside their organisms.

thus is very possible the existence of both scenarios, where populations of isolated individuals have different genetic traits, and share traits related to genetic mutation, due rats can easily travel overseas using human transport channels. so scientists could get a vast number of data, including human interaction, evolution of rodents, health development, spread of infections, development of new organisms, how genomes evolve in different environments and so on.