[TTPrograms]

Have we found diseases that could propagate across the world either (1) with virality greatly in excess of modern diseases, i.e. R0 >> omicron or (2) untreatable/curable and potentially latent for weeks to months?

A disease capable of coming close to ending civilization would need to have properties far beyond any disease observed so far. Either it needs to infect massive populations before we detect it, or it has to transmit over long distances (miles) despite e.g. moderate precautions like masking, air filtering. I think there's good reason to doubt such a pathogen could exist. The closest I could imagine would be an HIV-like immunodeficiency virus that can be transmitted via aerosol - but even that would have to cause disease much more severe than HIV without resistance among even .01% of the population.