"IT'S A MICROBE'S WORLD--WE'RE JUST LIVING IN IT."

Kyle Harper, The Fate of Rome: Climate, Disease, and the End of Empire (Princeton & Oxford: Princeton U. Press, 2017):

There are maybe a trillion microbial species in total; the average human lumbers around bearing some 40 trillion bacterial cells alone. They have been here for some three and a half billion years. It's a microbe's world--we're just living in it. Most of this wondrously diverse panoply is indifferent to us. There are only some 1400 microbes known to be pathogenic to humans. These have evolved the molecular tools--virulence factors--to menace us despite the defensive armory of our remarkable immune systems. The rise of a planer full of pathogens is very much the consequence of microbial evolution, which in turn has been profoundly determined by the explosion of human numbers and our species' pitiless transformation of landscapes across the globe. Evolution is propelled by the blind force of random mutation, but we have created the context in which evolution tinkers and experiments.

Id. at 291-292.