In the late 1800s, the German physician Robert Koch, in an effort to demonstrate that tubercle bacillus was responsible for tuberculosis, defined the set of criteria that must be present in order to determine a causal relationship between a microbial agent and an infectious disease. Koch’s Postulates held that the microbe had to be present in every case of the disease; it had to be specific; and scientists had to be able to isolate it, grow it, and introduce it into animals to cause a similar disease. And, for the past century or so, that has all pretty much held true.
“In this new era of molecular microbiology, however, Koch’s Postulates don’t work,” Dr. Lipkin said. “We’re not entirely sure yet what the new postulates will be. But we do know that there are microbes that we can’t grow in the laboratory, that not everybody infected with the same agent has the same manifestations, and that the sequelae may be long term.”
Dr. Lipkin points to instances of disease where the infection itself is not the problem, but the immune response of the infected host and the resulting damage—such as a virus or bacterium that manifests in immunosuppression, which may predispose to a variety of other disorders.