Smallpox is a viral disease unique to humans. To sustain itself, the virus must pass from person to person in a continuing chain of infection. It is spread by inhalation of air droplets or aerosols. It spreads most rapidly during the cool, dry winter months but can be transmitted in any climate and any part of the world.
Twelve to 14 days after infection, the patient typically becomes feverish and has severe aching pains. Two to three days later, a rash develops over the face and spreads to the extremities, and soon becomes pustular. Gradually scabs develop and eventually leave pitted scars. Death occurs in around 30% of cases. In 5% to 10% of smallpox patients, more rapidly progressive, malignant disease develops, which is almost always fatal within five to seven days.
Vaccination before exposure or within two to three days after exposure affords almost complete protection against the disease. Vaccination as late as four to five days after exposure may protect against death. There is no treatment for the disease. Besides vaccination, the only countermeasure to avoid spreading of the disease is quarantine.