I've been reading a bit on the Black Plague, which is as far as I understand one of the most (if not the most) significant epidemics in history. What I don't understand is how it ever died out. I understand that poor hygiene and crowded medieval cities led to millions and millions being infected and dying, but why did it just stop after a bit? Is there a reason why it stopped infecting people at the rate it had been doing so for the past several decades? Wikipedia doesnt mention this at all. Wouldn't a plague that wide-spread and uncurable just keep growing exponentially?
There has never been a definitive explanation as to why the Black Death subsided. After afflicting some seventy-five million to one hundred million people across Europe, Russia, and the Middle East, the disease began to taper off. Several factors are thought to have influenced the dramatic decline in fatalities caused by the Black Death.
The most popular theory of how the plague ended is through the implementation of quarantines. This entailed staying out of the path of infected individuals, rats, and fleas.
Part of it is the ongoing, slow replacement of the Black Rat (Rattus Rattus) by the stronger, more aggressive Brown Rat (Rattus Norvegicus). The Brown Rat's variety of flea is not very efficient at transmitting the plague to humans, while the flea of Rattus Rattus transmits the disease easily for some reason. The Brown Rat when it encounters Rattus Ruttus will kill or out-compete the other species, and has broken up its population into isolated pockets. In big cities where they both exist, Rattus Rattus lives in trees and roofs, whereas Rattus Norvegicus commands the sewers, the ground, and the better parts of the local ecosystem. Rattus Rattus thus tends to live in "islands" surrounded by areas populated by the competing rat.
It was the Black Rat that in the Middle Ages lived alongside humans and was a main vector of the plague. Apparently the Brown Rat was entering Europe at about that time, but the competitive replacement has been a slow process in human terms. While there are occasional people who get the plague from wild rodents of other species, the plague no longer can form an invasive species "wave" into a Black Rat or human population, especially in this age of better sanitation, antibiotics, and vector control. It can no longer transmit from rat to rat to people in a long, unbroken chain due to the increasing isolation of Black Rat populations.
There were many other factors, including the persecution of the house cat in the Middle Ages (though plague was a problem in Muslim lands too that were friendly to cats). Also, modern people are much less tolerant of rats.