Some thousands of years ago, for humankind as a whole, smallpox was an emerging disease, and it certainly followed this pattern during the European-American exchanges and African-American exchanges that followed the discovery of the Americas by Europeans.
There were several forms of the disease, with different case-mortality rates. Here I will describe only the commonest form (Figure 10.1). Infection occurred via the respiratory tract but there were no signs for 10 to 12 days after a person was infected and, during this incubation period, he or she was quite well. Then there was a sudden onset of fever, malaise, headache and backache (1 to 3 days),
followed by the development of a rash through stages of macules (2 to 4 days), progressing through vesicular, pustular and crusted stages over the following two weeks. All the lesions were at the same stage of development and there was a greater concentration on the face and limbs with a predominantly centrifugal distribution; each lesion was full of virus particles.
Smallpox was a specifically human disease, and the vast majority of infections resulted in disease with obvious clinical manifestations. Patients became infectious at the onset of fever and infectivity increased when the rash appeared, because lesions on the buccal mucous membranes broke down and virus was excreted in the saliva and pharyngeal secretions. Infectivity declined rapidly as these healed and the rash scabbed. Although there was a large amount of virus in the scabs, they were not an important source of infection. Recovery was followed by lifelong immunity and recurrences never occurred. These features meant that smallpox could be maintained as an endemic disease only in rather large populations, of the order of 200,000, so that small isolated populations—for example, of Amazonian Indians or the inhabitants of oceanic islands—never constituted a continuing focus of infection.
Infection occurred by close face-to-face contact, such as is common between members of the same household. Rarely, smallpox could be spread by fomites and, even more rarely, if the index case had a severe cough early in the course of the disease, infection could be airborne over moderate distances, within a bus or, rarely, a hospital, for example.