Most upvoted comment
Top rated history books on Reddit rank no. 46
If the Black Plague originally came from China and spread along trade routes, why do we only hear about the carnage it caused in Europe? Was China not affected by the plague?(r/AskHistorians)
Keys has a great chapter about Justinian’s plague in his book Catastrophe.
He subscribes to the theory that that plague in particular came from Africa, but the concept is the same. Basically, plague (Y. pestis) exists in several natural “reservoirs” around the world. These reservoirs consist of rodents such as wild gerbils that are: a) highly resistant to the effects of Y. pestis but nonetheless propagate the disease and b) are solitary and do not often come into contact with other susceptible rodents and/or primates.
The spreading of the disease of course comes from the fleas, who become infected when they feed on the infected members of the reservoir. Worse, the effect of the plague on fleas is that they can’t process food properly, so they panic and feed on everything in sight, hastening the spread of the disease.
So how do outbreaks happen? Well, during periods of extreme weather, the territory of these reservoir gerbils (or whatever) can be pushed into new areas. This may cause the spread of the disease to a more invasive species, like the black rat. Rats have no problem infringing on human living spaces, so when rats hitch a ride along trade routes, such as with goods coming from Africa to the Byzantine Empire during the time of Justinian, you see the spread of the disease to areas other than the source of the plague reservoirs themselves. To make matters worse, Justinian was working to reconquer the lost Western Roman Empire, and also dealing with the Persians under Khusrau at the same time, so that particular plague spread to those areas, too.
Note that it seems Justinian’s plague did in fact affect African port cities along the trade route, as well. I am less certain about the effects on China that the Black Death plague had, but this is nonetheless a relevant counterpart.