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.
Bookmarks