humans are not single organisms, but superorganisms made up of lots of smaller organisms working together.
The traditional view is that a human body is a collection of 10 trillion cells which are themselves the products of 23,000 genes. In every human being, and especially in his or her guts, dwells the microbiome: 100 trillion bacteria of several hundred species bearing 3m non-human genes. The bugs are neither parasites nor passengers. They are, rather, fully paid-up members of a community of which the human “host” is but a single (if dominating) member.
The other field that may be changed is genetics. Many of the diseases in which the microbiome is implicated seem to run in families. In some, such as heart disease, that is partly explained by known human genes. In a lot, though, most notably autism, the genetic link is obscure. This may be because geneticists have been looking at the wrong set of genes—the 23,000 rather than the 3m. For those 3m are still inherited. They are largely picked up from your mother during the messy process of birth. Though no clear example is yet known, it is possible that particular disease-inducing strains are being passed down the generations in this way.