Princeton University and Boston University researchers have developed a silicon wafer that can isolate cancer cells by separating them out according to size.

I don't have access to full text articles at new scientist but this was an idea I discussed with nano many years ago on a long car trip: Placing a nanotech filter in the blood stream to clean out circulating cancer cells. I figure it ought to be possible to do the same thing with virii...

KurzweilAI.net: Bloodstream Cancer filter