When did people with smallpox become contagious?

Individuals infected with smallpox were not considered contagious until the characteristic focal eruption appeared in the throat and on the skin.1

While the disease has an incubation period of one to two weeks following initial exposure, during which the patient shows no signs of infection, they do not spread the virus during this time. Even in severe cases where a toxic rash might develop during the initial fever, patients were generally not sources of infection unless the true smallpox rash had developed. Once the eruption occurred, the virus was typically transmitted through the inhalation of droplets expelled in the breath or saliva of the infected person.1