Families and households, too. If your home includes pets, they stretch the overlap out into the surroundings a bit more, kids mingle with other families pools of life, etc.<p>There's another layer of mites, molds, and so on that's multicellular and participates in our health but isn't counted here. The things that we don't see or think about, the things that fleas count as annoying little bugs.<p>Vicious jungles, full of life, red in tooth and claw, all the way down.