On 24 June 1763, William Trent, a local trader and commander of the Fort Pitt militia, wrote, "Out of our regard for them, we gave them two Blankets and an Handkerchief out of the Small Pox Hospital. I hope it will have the desired effect." The effectiveness of this effort to broadcast the disease is unknown. There are also accounts that smallpox was used as a weapon during the American Revolutionary War (1775–1783).According to a theory put forward in Journal of Australian Studies (JAS) by independent researcher Christopher Warren, Royal Marines used smallpox in 1789 against indigenous tribes in New South Wales. This theory was also considered earlier in Bulletin of the History of Medicine and by David Day. However it is disputed by some medical academics, including Professor Jack Carmody, who in 2010 claimed that the rapid spread of the outbreak in question was more likely indicative of chickenpox—a more infectious disease which, at the time, was often confused, even by surgeons, with smallpox, and may have been comparably deadly to Aborigines and other peoples without natural immunity to it. Carmody noted that in the 8-month voyage of the First Fleet and the following 14 months there were no reports of smallpox amongst the colonists and that, since smallpox has an incubation period of 10–12 days, it is unlikely it was present in the First Fleet; however, Warren argued in the JAS article that the likely source was bottles of smallpox virus possessed by First Fleet surgeons.
Smallpox was not notably infectious in the prodromal period and viral shedding was usually delayed until the appearance of the rash, which was often accompanied by lesions in the mouth and pharynx. The virus can be transmitted throughout the course of the illness, but this happened most frequently during the first week of the rash when most of the skin lesions were intact. Infectivity waned in 7 to 10 days when scabs formed over the lesions, but the infected person was contagious until the last smallpox scab fell off.Smallpox was highly contagious, but generally spread more slowly and less widely than some other viral diseases, perhaps because transmission required close contact and occurred after the onset of the rash. The overall rate of infection was also affected by the short duration of the infectious stage. In temperate areas, the number of smallpox infections was highest during the winter and spring. In tropical areas, seasonal variation was less evident and the disease was present throughout the year. Age distribution of smallpox infections depended on acquired immunity. Vaccination immunity declined over time and was probably lost within thirty years. Smallpox was not known to be transmitted by insects or animals and there was no asymptomatic carrier state. Mechanism
Once inhaled, the variola virus invaded the mucus membranes of the mouth, throat, and respiratory tract. From there, it migrated to regional lymph nodes and began to multiply.