Council of Twilight (page1)

Time is like a living being, and every action you take outside of your proper time is a virus infecting it. If your changes are small, the immune system will soon clean up the effects, and it will be as if you had never traveled through time at all. But a virus takes over the body's cells to replicate; to eliminate it, the immune system must destroy all the infected cells. And time's immune system is very aggressive; if it can't keep up with the virus's spread, it will nevertheless keep trying until there aren't enough cells left to keep the body alive.