It becomes communicable from the time of appearance of the first sores until all scabs from the sores have disappeared, which could be two or three weeks. The disease is spread by droplets in aerosol form. Usually about 20 to 40 percent of the cases are fatal, though modern intensive care may reduce mortality rates. Smallpox is unique. According to Peter Katona, assistant professor of medicine at UCLA who works with the Los Angeles health department on bioterrorism issues, one case would be an epidemic.
Smallpox is extremely highly contagious.
You don't get one case. There's nothing like smallpox. It's incredibly contagious. It's an airborne pathogen. Patients would need to be placed in isolation, preferably in a room with a negative air pressure, to avoid dispersal.