Hawking: Unintelligent life is likely on other planets
Is this somehow extrapolated from reading celebu-blogs and calculating outward into the universe?

As is so often the case (remember the measles, pox and our American indigenous peoples?) a the sum of our fears lies in the cooties*:
So should you worry about aliens? Alien abduction claims come from "weirdos" and are unlikely. However, because alien life might not have DNA like us, Hawking warned: "Watch out if you would meet an alien. You could be infected with a disease with which you have no resistance."

*In this case, cooties which might not respond to a bottle of Clorox!