I have long known that bacteria are more than 50% of the biomass of the planet. That was shocking enough when I learned it. Howsever, learning that the majority of cells in my body are not my own was something new to me. Even the fact that we cannot live without many species of bacteria did not surprise me. However, the idea that our own cells are outnumbered 10 to 1 within our own bodies was completely new to me. Check out this article from the New York Times Science section regarding this new and fascinating study.
If Cells Could Vote, People Would Be Minorities In Their Own Bodies