Half of you isn't you

For years people repeated that your body holds ten bacteria for every human cell. The real figure, calculated more carefully, turns out to be far less spectacular: about one to one.

A study revisited the numbers and estimated that an adult has some 30 trillion human cells and some 38 trillion bacteria, mostly in the gut. The ratio is around one to one, not the ten to one of the old myth.

Even so, the underlying idea remains unsettling: you carry nearly as many cells that aren't yours as ones that are. You're not an isolated individual, but a walking ecosystem that bargains every day with trillions of tenants.

Source: PLOS Biology