You never get something for nothing, especially not in health care. Every test, every incision, every little pill brings benefits and risks.

Nowhere is that balance tilting more ominously in the wrong direction than in the once halcyon realm of infectious diseases, that big success story of the 20th century. We have had antibiotics since the mid-1940s — just about as long as we have had the atomic bomb, as Dr. Martin J. Blaser points out — and our big mistake was failing long ago to appreciate the parallels between the two.

Antibiotics have cowed many of our old bacterial enemies into submission: We aimed to blast them off the planet, and we dosed accordingly. Now we are beginning to reap the consequences. It turns out that not all germs are bad — and even some bad germs are not all bad. In “Missing Microbes,” Dr. Blaser, a professor at the New York University School of Medicine, presents the daunting array of reasons we have to rethink the enthusiastic destruction of years past.

First and foremost, the war has escalated. Imprudent antibiotic use has resulted in widespread resistance among microbes; infectious disease doctors (I am one, as well as a casual acquaintance of Dr. Blaser’s) now operate in a state of permanent near panic as common infections demand increasingly powerful drugs for control.