How does a disease like the bubonic plague or a particular flu strain disappear and then reappear suddenly? Once it’s disappeared from a community, how can it then survive and show up again?
In: Biology
There are natural reservoirs of diseases – host populations where they survive: for plague in the US and Central Asia it’s all sorts of ground dwelling mammals like prairie dogs and such; for flu it’s ducks/pigs in China; for Ebola it’s various primate populations in west/equatorial Africa.
Herd immunity.
If the disease persists long enough, people eventually develop a resistance to it. But not all people have immune systems strong enough to eradicate the disease. So what happens is that, after the pandemic, people just keep living, wile the pathogen stays in people that are not quite strong enough to kill it, but strong enough to not let it burst. So even if it did spread, it would get instakilled by the immune systems of other people. Then, because the following generations are not exposed to the pathogen directly, the resistance to it decreases, until it reaches a point where the immune systems of other people can no longer recognize and fight it immediately, hence pathogen can thrive again, and the process repeats itself.
This cycle takes about 4 generations, with the first generation having the strongest immune systems, and the 4th being the weakest. This is why we have a pandemic roughly every 100 years. 100 years is enough time for the general immunity to get lazy and allow disease to roam free.
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