Why did the Plague Stop?

Someone asked us this question, “Why did historical diseases like the black death stop?” The answer is that people still get bubonic plague in some parts of the world, just not on an endemic scale. Perhaps because of sanitation, it doesn’t spread like it used to, but it has not completely been eradicated.

In general, infectious diseases never keep going forever at a high rate, because as soon as the number of healthy individuals drops significantly (due to infection, immunity, or death), transmission rate drops and so does the supply of fresh host to spread to. This is why deadly diseases only come in sudden waves and die down.

The key to controlling any infectious disease is to reduce the number of susceptible people by any means possible (vaccine, quarantine, or getting them all infected). It’s called the SIR model (Susceptible-Infected-Removed). It’s the basis for all modern infectious disease models.