Again, they described the elements of their model: the circulation phase, when the virus is spilling from one kind of animal host to another, remaining broadly adapted, capable of infecting them all; the first accident, when a genetic change happens to give the virus added advantage in humans; and the second accident, a societal circumstance that allows the virus to amplify and spread, approaching a critical point. (This is like what Malcolm Gladwell described in his book The Tipping Point.
