As bad as things were for some hospitals (my mom worked as a medical coder in the Phoenix area, so I heard horror stories), and as annoying as it was to not be able to find toilet paper and ground beef, what I'm talking about is different. If a virus were to exist that, for example, spread like the measles (R0 as high as 18) and killed like ebola (50%+), things are going to deteriorate well beyond overcrowded hospitals and using napkins to wipe your butt. In a short amount of time, most of the population will be infected. We aren't going to ease into her immunity, like we did with Covid. We are going to crash into herd immunity. If things were to get bad enough, then you start to worry about the supply chain and whether or not grocery store employees are even going to come to work. Do police come to work? What about the people who run the power grid? Things could deteriorate very, very quickly.
As I mentioned, to our knowledge there hasn't been such a virus occurs naturally. At least not one that infects humans. But with advancements in synthetic biology, combined with incredibly dangerous gain of function research, that possibility is real.