Unfortunately we don’t need bizarre externalities to explain plagues, or any other product of evolution. In this case Occam’s Razor truly applies. There are some more probable externalities that may apply in the context of modern diseases that may (or may not) have originated in a lab. In other words, “we can do bad all by ourselves”. That doesn’t necessarily preclude the possibility of the kind of radical external intervention insinuated by OP, by my guess is the math would not be in favor of this probability, at least as far as plagues go. Genetic alterations periodically? Again Occam’s Razor would suggest otherwise, but I remain receptive to the possibility.