Dr_Ken writes with a quote from Scientific American:
"The human body has some 10 trillion human cells—but 10 times that number of microbial cells. So what happens when such an important part of our bodies goes missing? With rapid changes in sanitation, medicine and lifestyle in the past century, some of these indigenous species are facing decline, displacement and possibly even extinction. In many of the world's larger ecosystems, scientists can predict what might happen when one of the central species is lost, but in the human microbial environment—which is still largely uncharacterized—most of these rapid changes are not yet understood. 'This is the next frontier and has real significance for human health, public health and medicine,' says Betsy Foxman, a professor of epidemiology at the University of Michigan School of Public Health in Ann Arbor. Meanwhile, each new generation in developed countries comes into the world with fewer of these native populations. 'They're actually missing some component of their microbiota that they've evolved to have,' Foxman says."