In addition to what the other commenter said, there’s some luck of the draw, too. There were three forms of it, having to do with how you were infected. Bubonic was one, associated with sores and boils on the skin, caused by flea bites. Pnumonic was a lung infection, which could spread directly via droplets. And septemic was the blood infection version, usually happening as one of the others progressed.
Bubonic only killed about 40-60% of those who showed symptoms, while pnumonic and septemic killed 90-100% of those who showed symptoms.
So to get infected at all, you needed either to be bitten by an infected flea, share air with someone who has pnumonic, or share fluids with someone that has bubonic (specifically the pus from the sores) or septemic (the blood, though maybe other fluids too).
Some managed to avoid these entirely. Others could have had lower exposures to the point where they didn’t develop symptoms. If someone gets infected but the infection doesn’t get established enough to become stable, they often don’t get treated any differently from people who weren’t infected at all. Those death rates only apply to those that they knew had it (though sometimes death rates are given per population rather than infected, and those tended to vary wildly in infected areas, from like 50% to 80%).
With viruses, at least, asymptomatic infection seems to be far more common than we would have thought. Both ebola and covid antibody studies showed that the antibodies were found in many who never got sick, implying they were exposed but their immune system beat it before symptoms showed up.
Bacteria isn’t necessarily the same, but it’s possible that something like this is a factor and those might have even developed some immunity. Plus, natural selection would select for people who are just less susceptible to it while it’s out there killing off a significant part of the population.
SkunkWorkz@lemmy.world 5 weeks ago
Some people didn’t get it and some had the right genes to fight off the disease. Those genes have now been linked to autoimmune diseases …harvard.edu/…/genes-protective-during-the-black-…