the premise is wildly disengenious in conflating the pureification of water supplys, and air. a simple set of facts is that humans now represent the largest mass and population of large animals ever to exist. this population inhabits many habitats , both natural and constructed. this population has huge numbers of indivuals who have compromised imune systems. and last, but worst is that humans now constitute a biological niche, and a primary law of biology is the "all niches will be filled"