What is the probability of three/four viruses combining?

Americans thought that the day they got a black president, pigs would fly. Three months later, swine flu!.....

All joking aside, we are told that swine flu has components from four other viruses (although I had heard it was three), and a quick search on google produces even more confusion.

Though I’m no virologist (by any means) I am skeptical of how this can come about. Just wondered if anyone could shed any light on the subject, or had an opinion, or do you think big pharma is justifying its own existance, or worse…...