Tbf Covid was unique in that it was a virus capable of airborne transmission while being asymptomatic and also it can be fatal without killing too quickly. That's extremely unusual. Even more rare, some people never showed symptoms or the symptoms were so mild so it was unclear if it was Covid at all.

Hemorrhagic fevers (including Ebola), for example, are incredibly deadly but that really limits their ability to spread. You still have to be diligent but there's a reason why Ebola has been contained to parts of Africa the the virus seemingly spreads from animals to humans every now and again, starting another local outbreak. But it's never spread globally and it's unlikely to.

Probably the last comparable disease to Covid was the Spanish flu in terms of its ability to spread and that was over 100 years ago. It's worth noting that the Spanish flu could also spread asymptomatically, which also explains it's impact to a large degree. Pretty much every modern influenza variant is a derivative of the Spanish flu, which also tells you something about what tends to happen to novel diseases.

Y pestus is a bacteria, not a virus. We now have antibiotics that could likely combat it, which was obviously something that didn't exist in medieval Europe. Back then it wasn't at all clear that diseases spread by microorganisms at all. Plus, rodents were key to that spread and that's a far smaller problem now.

The nightmare scenario is probably some deadly variant of measles because measles is so incredibly contagious, probably more than any other disease I can think of. It can survive airborne for a very long time and in the modern era we've traced outbreaks of tens of thousands of cases to one single person. But the ability of measles to survive outside a host is also (luckily) incredibly rare.

> ”Pretty much every modern influenza variant is a derivative of the Spanish flu”

Not really. While it was true for much of the last century that many “seasonal flu” viruses were direct descendents from H1N1 Spanish flu, the H1N1 that caused the 2009 “swine flu” pandemic was a distinct strain combining new genes of pig, avian, and human flu origins. Since then, derivatives of that virus have replaced earlier ones as the most common circulating H1N1 types.

And right now the most common circulating influenza is actually H3N2 from a completely different (avian) lineage with origins in the 1968 pandemic.

Unfortunately, you can force evolution in bacteria, fungi and viruses. There's more than enough examples of multi-resistant pathogens in all three families, especially in hospitals but also around agriculture (because they use stuff meant as a last-resort to promote animal growth or prevent animals from falling ill to the effects of overcrowding)... honestly between these and prions, I often do wonder how long we as humanity will be able to keep both hospitals and large-scale farming in existence.