In the 1990s, geneticists hoped that mapping the human genome would provide a path to treating many seemingly incurable or poorly understood conditions; obesity, dementia, and cancer, to name a few. As understanding of genetics expanded and improved, however, scientists came to see the genome as just a first step in understanding a vastly more complex world of interactions within the body, one that extended to organisms that were not traditionally considered native or essential to humans—the microbiome.

Research at Yale focuses on a wide swath of relationships between microbes and humans, from the gut to DNA. According to the latest inquiries, the evolution of Homo sapiens is not just a tale of vertebrate mutations, in which genetic variation leads to useful adaptation, but a hyper-complex series of mutually beneficial arrangements between the human body and the many organisms that call it home.

In some cases, researchers are still attempting to establish a correlation between a given microbe and conditions in humans. This issue explores some of those possibilities in articles about diabetes, cystic fibrosis, and multiple sclerosis. In other cases where a correlation has been established, such as with Clostridium difficile infections, the question is not whether microbes play a role in human health, but the specific nature of that role.

One fact has become apparent in compiling this issue of Yale Medicine Magazine: the old idea of the human body as a self-sufficient entity constantly fighting hostile microbes is obsolete. We are collections of organisms working together to survive.

At least on a cellular level, the concept of the heroic individual is not just anomalous—it is absurd, unworkable. Each person is a miracle of cooperation and altruistic mutual interest, a collective of creatures. Good health and evolutionary success are the products of alliances, not ruthless selfish competition.