Most bacteria are well-behaved companions. Indeed, if you are ever feeling lonely, remember that the trillions of microbes living in and on the average human body outnumber the human cells by a ratio of 10 to one. Of all the tens of thousands of known bacterial species, only about 100 are renegades that break the rules of peaceful coexistence and make us sick.
Collectively, those pathogens can cause a lot of trouble. Infectious diseases are the second leading cause of death worldwide, and bacteria are well represented among the killers. Tuberculosis alone takes nearly two million lives every year, and Yersinia pestis, infamous for causing bubonic plague, killed approximately one third of Europe’s population in the 14th century. Investigators have made considerable progress over the past 100 years in taming some species with antibiotics, but the harmful bacteria have also found ways to resist many of those drugs. It is an arms race that humans have been losing of late, in part because we have not understood our enemy very well.