now? like you think deadly virus are new to human population? that this is not the natural order as it has been since the beginning of time? anything like ebola with a 80 or 90% kill rate is not going to spread like covid. the people die before it spreads.
Not if it only kills after a long asymptomatic (but infectious) incubation period.
I guess the nightmare case would be something with the infectiousness of coronavirus, the deadliness of Ebola, and the incubation/asymptomatic period of HIV. The whole world could be infected before people start to show symptoms (then everyone dies).
Either that, or zombies (which would allow it to spread widely despite being quickly fatal).
There presumably is a reason why such things have not generally occurred in our history (or that of other large species)? Otherwise infectious organisms would have 'won' long ago, and all larger species been obliterated.