How Is It That Scientists Didn't Know Until Fairly Recently

How is it that scientists didn't know until fairly recently that humans carry around whole ecosystems of bacteria?

A) We didn't have strong enough microscopes to see bacteria.
B) They weren't looking for them.
C) The bacteria don't grow well in the lab, and only lately have we had the technology to identify bacterial species by RNA.
D) The bacteria don't grow well in the lab, and only lately have we had the technology to see them.
E) Bacteria have especially complex RNA, so it took awhile use that to identify species.