This is another great video explaining our human microbiome and what we do to it, generally terrible things.
This explain simply the fact that we are 1 part human and 10 parts microbes, based on the number of cells in our body and the number of microbes in and on our body.
We need these, we have co-evolved with the microbes. Without them we are susceptible to many things, we are unable absorb nutrients etc.
The graphics are fantastic.