Finlay et al in Scientific American:
Until very recently, whenever we thought of microbes — especially around babies — we considered them only as potential threats and were concerned with getting rid of them, and it is no surprise why. In the past century, most human communities have experienced the benefits of medical advances that have reduced the number and the degree of infections we suffer throughout life. These advances include antibiotics, antivirals, vaccinations, chlorinated water, pasteurization, sterilization, pathogen–free food, and even good old-fashioned hand washing. The quest of the past 100 years has been to get rid of microbes — “the only good microbe is a dead one.” This strategy has served us remarkably well; nowadays, dying from a microbial infection is a very rare event in developed countries, whereas only 100 years ago 75 million people died worldwide over a span of two years infected with H1N1 influenza virus, also known as the Spanish flu. At first glance, our war on microbes along with other medical advances has truly paid off. The average life span in the US in 1915 was 52 years, approximately 30 years shorter than it is today. For better or for worse, there are almost four times more humans on this planet than only 100 years ago, an incredibly accelerated growth in our historic timeline. Evolutionarily speaking, we have hit the jackpot. But at what price?