One of the techniques modern factory farms routinely use to increase weight in livestock is to give all of the animals a dose of antibiotics with every meal. When this is done, the bacteria in the animals’ guts that are susceptible to the drugs are killed. When this practice is ongoing, it creates a microbial vacuum in the animals’ intestines that gives an extraordinary competitive advantage to any bacteria that develop resistance to the antibiotics. If your goal was to breed bacteria that could not be controlled by antibiotics, you could hardly design a more effective system. It is not entirely an exaggeration to say that as a result, factory farms have become biological weapons factories. Antibiotics have saved millions of lives, and their discovery ranks with the great medical achievements of history. But even Sir Alexander Fleming, the man who first discovered penicillin, warned that overuse of the drug would lead to bacterial resistance. And indeed, the drugs have been heavily overused, with increasingly alarming consequences. This year, between 70,000 and 100,000 Americans will die from infections that could once have been cured with common antibiotics.