Wired News: People Are Human-Bacteria Hybrid emphasizes just how many “friendly aliens” share our bodies with us:
Most of the cells in your body are not your own, nor are they even human. They are bacterial. From the invisible strands of fungi waiting to sprout between our toes, to the kilogram of bacterial matter in our guts, we are best viewed as walking ‘superorganisms,’ highly complex conglomerations of human, fungal, bacterial and viral cells. [...] More than 500 different species of bacteria exist in our bodies, making up more than 100 trillion cells. Because our bodies are made of only some several trillion human cells, we are somewhat outnumbered by the aliens. It follows that most of the genes in our bodies are from bacteria, too.