People are only infectious after the rash appears, and remain infectious for about 2 weeks. Transmission usually requires prolonged face-to-face contact with an

People are only infectious after the rash appears, and remain infectious for about 2 weeks. Transmission usually requires prolonged face-to-face contact with an infected person. Prior to the rash appearing, the infected person will feel lethargic and ill, and will likely take to bed. So if there were a potent enough exposure event, it would be fairly easy to recognize and contain. Smallpox wouldn't make a very effective biological weapon (except for psychological effect).