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Smallpox is an infectious disease unique to humans, caused by either of two virus variants, Variola major and Variola minor.[1] The disease is also known by the Latin names Variola or Variola vera, which is a derivative of the Latin varius, meaning spotted, or varus, meaning “pimple”. The term “smallpox” was first used in Europe in the 15th century to distinguish variola from the great pox (syphilis).[2]
Smallpox localizes in small blood vessels of the skin and in the mouth and throat. In the skin, this results in a characteristic maculopapular rash, and later, raised fluid-filled blisters. V. major produces a more serious disease and has an overall mortality rate of 30–35%. V. minor causes a milder form of disease (also known as alastrim, cottonpox, milkpox, whitepox, and Cuban itch) which kills ~1% of its victims.[3][4] Long-term complications of V. major infection include characteristic scars, commonly on the face, which occur in 65–85% of survivors.[5] Blindness resulting from corneal ulceration and scarring, and limb deformities due to arthritis and osteomyelitis are less common complications, seen in about 2–5% of cases.
Smallpox is believed to have emerged in human populations about 10,000 BCE.[2] The disease killed an estimated 400,000 Europeans each year during the 18th century (including five reigning monarchs), and was responsible for a third of all blindness.[3] Between 20 and 60% of all those infected—and over 80% of infected children—died from the disease.[6]
During the 20th century, it is estimated that smallpox was responsible for 300–500 million deaths.[7][8][9] In the early 1950s an estimated 50 million cases of smallpox occurred in the world each year.[10] As recently as 1967, the World Health Organization estimated that 15 million people contracted the disease and that two million died in that year.[10] After successful vaccination campaigns throughout the 19th and 20th centuries, the WHO certified the eradication of smallpox in December 1979.[10] To this day, smallpox is the only human infectious disease to have been completely eradicated.[11]
Smallpox is caused by infection with variola virus, which belongs to the genus Orthopoxvirus, the family Poxviridae, and subfamily chordopoxvirinae. Variola virus is a large brick-shaped virus measuring approximately 302 to 350 nanometers by 244 to 270 nm,[12] with a single linear double stranded DNA genome 186 kilobase pairs (kbp) in size and containing a hairpin loop at each end.[13][14] The two classic varieties of smallpox are variola major and variola minor. The closest viral relative is molluscum contagiosum, which, like smallpox, infects only humans. However, unlike variola species, molluscum infection is benign.
The lifecycle of poxviruses is complicated by having multiple infectious forms, with differing mechanisms of cell entry. Poxviruses are unique among DNA viruses in that they replicate in the cytoplasm of the cell rather than in the nucleus. In order to replicate, poxviruses produce a variety of specialized proteins not produced by other DNA viruses, the most important of which is a viral-associated DNA-dependent RNA polymerase. Both enveloped and unenveloped virions are infectious. The viral envelop is made of modified Golgi membranes containing viral-specific polypeptides, including hemagglutinin.[13] Infection with either variola major or variola minor confers immunity against the other.[4]
Four orthopoxviruses cause infection in humans: variola, vaccinia, cowpox, and monkeypox. Variola virus infects only humans in nature, although primates and other animals have been infected in a laboratory setting. Vaccinia, cowpox, and monkeypox viruses can infect both humans and other animals in nature.[15]
Transmission of smallpox occurs through inhalation of airborne variola virus, usually droplets expressed from the oral, nasal, or pharyngeal mucosa of an infected person. It is transmitted from one person to another primarily through prolonged face-to-face contact with an infected person, usually within a distance of 6 feet, but can also be spread through direct contact with infected bodily fluids or contaminated objects (fomites) such as bedding or clothing. Rarely, smallpox has been spread by virus carried in the air in enclosed settings such as buildings, buses, and trains.[16] The virus can cross the placenta, but the incidence of congenital smallpox is relatively low.[4] Smallpox is not notably infectious in the prodromal period and viral shedding is usually delayed until the appearance of the rash, which is often accompanied by lesions in the mouth and pharynx. The virus can be transmitted throughout the course of the illness, but is most frequent during the first week of the rash, when most of the skin lesions are intact.[15] Infectivity wanes in 7 to 10 days when scabs form over the lesions, but the infected person is contagious until the last smallpox scab falls off.[17]
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