I suspect that the greatest anthropogenic extinction threat in this century is the possibility of a natural pathogen engineered (perhaps somewhat accidentally) to spread easily from person to person, persist in the environment, resist antibiotics and immune responses, and cause 100% mortality without evolving toward less virulence. I would like to hear expert opinion on whether evolutionary pressure toward optimal virulence has kept pathogens from exploring the most dangerous parts of the virulence landscape. I'm hoping that pathogens spread by intermediaries (e.g., vectors and wastes) have had little incentive to avoid those parts of the landscape, and that mad scientists will thus not find any magic bullets there. But just as certain well-known technologies (e.g. the wheel, internal combustion) seem to be unreachable in evolution's search space, I worry that there are techniques for increased virulence that a mad scientist could find much more easily than evolution ever could.
I also worry that I'm simply ignorant of the heights of virulence that evolution has already been able to reach. Is there a survey of the worst known and suspected cases of pathogenic (near-)extinction in humans, mammals, and animals in general? For example,
- Variola major and a couple other pathogens were together able to kill somewhere between 25% and 95% of native Americans as a result of the Columbian Exchange -- including 100% of the population of Hispaniola.
- The enterobacterium Yersinia pestis in the 14th century killed up to 1/3 of Eurasia's population.
- The Spanish Flu of 1918 killed up to 5% of the world's population.
As horrific as these and other pandemics have been, they are obviously not as bad as extinction. My casual searching on the web found no discussion of continent- or planet-wide animal extinctions caused by pathogens, but I'm skeptical that there haven't been any.