Smallpox is an extremely contagious eradicated disease caused by the virus Variola minor or Variola major.


Before it was eliminated smallpox had been one of the most spread infection diseases in the world. For the first time the term “smallpox” was used in the 15th century in England to show the difference between this disease and the “great pox” – syphilis. People who had smallpox developed fever and a progressive rash. Around 3 out of 10 affected persons died. The disease though had left visible permanent scars over large areas of the body. The preventive measures organized by the WHO helped to eliminate the infection and since 1977 no case of smallpox has been reported. Nowadays only a small amount of the virus is being kept in the laboratories in the USA and Russia, where the scientists continue researches. Smallpox is thought to be one of the possible biological warfare along with anthrax, plague, botulism etc. Only two smallpoxinfectious diseases are considered eradicated: smallpox and rinderpest.


The disease is caused by a poxvirus called variola that belong to the genus Orthopoxvirus. The virus contains double-stranded DNA and can be found all over the body of the affected person.

Risk factors

Smallpox is a contagious disease that spreads via respiratory route and by the direct person-to-person contact, although the indirect passage is also known. Now only the scientists who work with the virus are at risk at some point.


Variola major is the severe and the most common form of the disease, while variola minor is a lighter form and very rare. In vaccinated persons smallpox without rash was observed. This form is called variola sine eruption.
Typically smallpox involves the occurrence of different symptoms on several stages of the disease’s progression.

1 – Incubation period

Lasts from 7 to 19 days (the average length -10-14 days). This period begins when a person comes into contact with the causative agent until he feels himself ill. During this period the person is not contagious. The virus invades the mucous membrane of the respiratory tract and mouth, and then moves to the regional lymph nodes where it multiplies. After several days the virus gets into the bloodstream and viremia occurs. The viral particles spread in the body and multiply in the spleen, bone marrow and lymph nodes.

2 – Initial symptoms

The stage lasts 2-4 days, when a person develops high fever, headache, muscle pain, malaise, nausea and vomiting. Usually an affected person is not contagious at this point.

3 – Early rash

Lasts 4 days. The person is most contagious. On this stage appears the rash. It typically begins as small red spots on the mucosa of the tongue, palate, throat and the mouth – enanthem. Later the spots change into the sores, which rupture and release the virus into the oral cavity and throat. By this time the temperature decreases to normal.

After 24-48 hours occurs a rash on the skin. . Usually it starts on the face then spreads to the arms and legs and finally hands and feet. It takes around 24 hours for the rash to spreads all over the body’s surface. This rash is called maculopapular.
The rash is the densest on the face and limbs, it could have also been found on the palms of the hands and soles of the feet.

Until the 4th day the sores get filled with a thick fluid that looks like a pus and the blisters are formed. The fever rises again and remains high.

4 – Pustular rash and scabs

The stage lasts for 10 days.

During this period the sores change into pustules – raised blisters, firm to the touch. After 5 days they start to form crusts and scab. Sometimes, the blisters merge into sheets and form a confluent rash, which begin to detach the outer layers of skin from the underlying flesh.

5 – Scabs fall off

It takes about 6 days for the scabs to fall off. The spots, marks and even depigmented scars remain on the skin.

6 – When there are no more scabs a person is considered not contagious.
In 2-5% of cases the disease have led to corneal ulceration, scarring and as a result – blindness. Arthritis and osteomyelities that occurred as a complication caused limb deformities in several cases.

Some people developed hemorrhagic smallpox – a form associated with the extensive bleeding into the gastrointestinal tract, mucosa and the skin. The hemorrhages made the skin look black and gave the name to this form – “black pox”. Usually this form is lethal to the infected person.


The diagnosis can be verified by the means of skin biopsy. Poxviruses produce characteristic cytoplasmic inclusions, which are detected microscopically when the hematoxylin and eosin stains are used. The most important of these inclusions are known as Guarnieri bodies, and are the sites of viral replication. The virus may be grown chorioallantoic membrane (part of a chicken embryo). The strains are identified with the help of the polymerase chain reaction (PCR). Serologic tests and enzyme linked immunosorbent assays (ELISA) measure variola virus-specific immunoglobulin and antigen.

Treatment and prevention

The infection was eradicated with the help of the smallpox vaccine. The smallpox vaccine may induce toxic or allergic reactions and even some life-threating (very rare) – encephalitis or severe necrosis in the vaccination’s area.
The treatment for the disease is not surely known. The symptomatic treatment was usually used to take care of the wonds, control the fluid balance, infection and main vital signs. Tecoviramat, Cidofovir and Brincidofovir were effective in laboratory tests, but they’ve never been used for the treatment of the infected person, so the effect could not be predicted.

[See also related: Yaba poxvirus disease]

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