Quote Originally Posted by Seamus Fermanagh View Post
It would be bad, but likely less bad than the flu outbreak at the end of WW1. Our ability to support patients and mitigate symptoms allows us to minimize death-rates from any naturally occurring virus. Not that it still wouldn't suck, just saying that a Black Death levels of casualties are unlikely outside of Central Africa or Indonesia.

Particularly deadly viruses tend to burn out faster as the hosts' deaths minimize the ability of the infection to spread. Viruses that have mutated to insure massive spread (like Rhinoviruses) tend to be less virulent.

It would take a multi-layer engineered virus to have the kind of earth-changing effect you fear.
It's worth pointing out that as bad as the Black Death was it didn't actually cause societal collapse - nor did the Flu, even the Plague od Justinian was a one-two-three punch with sudden climate change and external invasions.

On the other hand, so long as humanity continues to overpopulate the planet we are playing Russian Roulette with disease - eventually one will come along that does cause societal collapse.