Yeah, I was gonna say early Climate Change effects and Automation/AI's wealth concentration. Just wait until the Genetic Engineering really starts rolling now that we have CRISPR-CAS9.
Meh, mortality rate would be bad, but not as bad as it was in those days. IVs help a lot with dehydration, which they did not have. You'll need something more lethal to accomplish the terrifying and sadistic goal of being the worst thing to have happened to the world since the dinosaurs. Likely, we will just mumble along like we always have through the black death, polio, etc.