During the Gulf War there was some guy on a radio show, maybe Art Bell (I know, I know), that went through several scenarios that Sadaam could use to strike against the US.
One included using a few people (less than 30) who were infected with smallpox or some manipulated strain of influenza, then when they approached the point that the disease was at its most contagious, putting them on planes to criss-cross the country.
I just remember it because it would be cheap and effective if you could find folks to do it.
Wasn't there some Bruce Willis movie that had the same theme... I can vaguely remember seeing it... Oh well.