It has been calculated that the normal human houses about a trillion bacteria on the skin, 10 billion in the mouth, and 100 trillion in the gastrointestinal tract. The latter number is far in excess of the number of eukaryotic cells in all organs which comprise the human host. It is sometimes said quite simply that there is more of "them" than "you'' in you. The normal flora occupy available colonization sites which makes it more difficult for other microorganisms (nonindigenous species) to become established.Eeeeeeww!!!! (Link) At least those little bastards serve a good purpose.
Also, from the same article, did you know that Streptococcus bacteria can survive in space? Craziness!