Your body has about ten times the number of microbial cells as it does human cells. Collectively this bacteria weighs about three pounds, about the same as your brain. The evolution of our understanding of these bacteria has been evolving rapidly over the past twenty years, and I expect its findings to be the most impactful on our health and wellness in the coming decades.
The New Yorker has a fantastic article by Michael Specter that starts and ends with our changing perception of heliobacter pylori, linked to gastritis and peptic ulcers, but whose absence in children (it used to be universal, now fewer than 5% of children in the US carry it in their guts) is linked to asthma. It also has consequences for weight: