The covid-19 pandemic (which I’m hoping will come to and end soon, I’m getting pretty tired of it) has been a bit different from what most people think when they imagined a deadly worldwide pandemic. They think of something like the Black Death, that may have killed 60% of everybody in Europe. A disease that kills 1% of the people who get it, and even less if you’re young with no preexisting conditions, doesn’t feel quite so serious. And considering the huge societal impact the response to it had, it’s honestly understandable that many people felt the response was out of proportion to the seriousness of the disease.
But when it comes to pandemics this is actually a bad thing. A deadlier disease can end up killing fewer people, even with all else being equal. Consider Ebola, which is far deadlier than covid, killing roughly 50% of those who get it. But its worst outbreak killed fewer people worldwide than covid killed in a single week in the US in January.
It makes sense if you think about it, as a dead person isn’t going to spread a disease to very many people. In fact from an evolutionary perspective killing people is the last thing a virus wants to do. A virus that kills people isn’t going to spread as much as one that doesn’t. But when it comes to pandemics, it needs to kill at least some people for us to take notice and take it seriously. To be a pandemic the disease needs to be deadly enough to cause a high number of deaths, but not so deadly that the people who have it won’t spread it around.
Covid has threaded that needle by not being all that deadly on an individual level (and therefore spreading to millions of people) but being deadly enough that it takes out a small fraction of that number. A small fraction of a large pie is still a large number, especially when it comes to a serious side effect like death. This is why half a million people in the US have died of covid, a disease that has a 99% survival rate, while only a single person in the US has died from Ebola, a disease that kills half the people who get it.