How Smallpox Spreads

How it spreads

Person to person

Human-to-human transmission of variola virus occurs by inhalation of large, virus-containing airborne droplets of saliva from an infected person. Before smallpox was eradicated, it was mainly spread by direct and fairly prolonged face-to-face contact between people.

Smallpox patients became contagious once the first sores appeared in their mouth and throat (early rash stage). They spread the virus when they coughed or sneezed and droplets from their nose or mouth spread to other people. They remained contagious until their last smallpox scab fell off.

Smallpox can be spread by humans only. Scientists have no evidence that smallpox can be spread by insects or animals.

Scabs

Scabs and the fluid found in the patient's sores also contained the variola virus. The virus can spread through these materials or through the objects contaminated by them, such as bedding or clothing. People who cared for smallpox patients and washed their bedding or clothing had to wear gloves and take care to not get infected.

Airborne

Rarely, smallpox has spread through the air in enclosed settings, such as a building (airborne route). Though rare, there have also been reports of airborne transmission in hospital and laboratory settings. Adhering to infection control procedures can reduce this risk.

When the disease can be spread

Smallpox patients are considered infectious from the time the first sores appeared in the mouth or throat (oropharyngeal lesions), throughout the course of the disease, until the last scab falls off the body.

Understanding risk

A person is considered at risk for contracting smallpox after prolonged, close contact with an infectious smallpox patient. Before smallpox was eradicated, the disease had a secondary household or close contact attack rate of approximately 60% among unvaccinated individuals.