You sure about that?
I've read that smallpox is contagious during incubation, prior to the manifestation of symptoms. There were also several smallpox threads on the subject last week, with comments from doctors, and none suggested that the virus wasn't contagious prior to the development of pustules.
I'm not an expert, but based on what very little I know, I would guess that there is probably at least a very narrow window of opportunity between the moment when the earliest symptoms develop and the earliest moment when the symptoms would be obvious enough to make smallpox a suspicion. In other words, during those first few hours when you start coming down with something, you usually don't know for sure whether it is just a cold, the flu, or something more serious. It is that time period when a contagious kid could infect dozens at school, maybe more dozens on the school bus, the rest of the family, then dozens more in the waiting room. All within the course of perhaps three or four hours.
I believe (and I'll admit it if I'm found to be wrong) it behaves like chicken pox - incubation is several days to 2 weeks and then the pox show up. Once the pox show up is when one is contagious. That's why my pediatrician hustled me out of the waiting room and out the back door of the office when I brought my son in with this strange rash last year . . . .sure 'nuff - It was chicken pox (a year AFTER his chicken pox vaccination).
Actually, that is incorrect. According to the CDC:
Twelve to 14 days after infection, the patient typically becomes febrile and has severe aching pains and prostration. Some 2 to 3 days later, a papular rash develops over the face and spreads to the extremities). The rash soon becomes vesicular and later, pustular . The patient remains febrile throughout the evolution of the rash and customarily experiences considerable pain as the pustules grow and expand.
So there are initial symptoms which are likely to cause the person to become bedridden, BEFORE the rash appears. So there is no "window of opportunity".