Yong broadens the scope beyond human health to show how these partnerships define the natural world:

Some deep-sea creatures without mouths or guts rely entirely on microbes for energy.

The central thesis of Yong's work is that "individuals" do not exist in isolation. Every animal, from the Hawaiian bobtail squid to humans, is an "ecosystem on legs". We are teeming with trillions of microbes that outnumber or at least rival our own human cells, functioning as an interconnected, interdependent whole. This perspective shifts our identity from a single organism to a thriving, complex colony of life. Microbes as Biological Architects

Microbes can bombard their hosts with genes, effectively modifying the genetic makeup and evolution of the species they inhabit. The Human Impact