Actually, even a cursory glance at smallpox resources on the net shows that people aren't contagious until the rash starts, but it's sort of a tossup of whether they would be moving around or not. Someone in the throes of smallpox is fairly identifiable, and therefore easier to avoid.
All this goes out the window if they figured out a way to aerosolize the virus and infect hundreds or thousands of people all at once.
Could someone with partial immunity, a person who has been vaccinated, be infected and yet not so sick as to be noticed? There seem to be a lot of unknowns about smallpox.