They probably were clean samples, however this was a horribly negligent and incredibly arrogant stunt by Gates. He may have made his point, but a public apology should be the minimum repentance, if not some sort of gift to those who feel they were unfairly treated. I'm hardly one that sympathizes with those who instantly scream "victim", but this was pretty outrageous, unnecessary, and unacceptable.
Maybe not the brightest thing for Bill to do, but this fear of encephalitis and West Nile was an overreaction.
At most they could have been a carrier for diseases among people in the room.