The initial symptoms of the disease include fever and vomiting. This is then followed by formation of sores in the mouth and a skin rash. Over a number of days the skin rash turns into characteristic fluid filled bumps with a dent in the center. The bumps then scab over and fall off leaving scars. The disease used to spread between people or via contaminated objects. Prevention is by the smallpox vaccine. Once the disease has developed certain antiviral medication may help.
The origin of smallpox is unknown. The earliest evidence of the disease dates back to the 3rd century BC in Egyptian mummies. The disease historically occurred in outbreaks. In 18th century Europe it is estimated 400,000 people per year died from the disease, and one-third of the cases resulted in blindness. These deaths included those of at least five reigning monarchs. In the 20th century it is estimated that smallpox resulted in 300–500 million deaths. As recently as 1967, 15 million cases occurred a year.
Edward Jenner discovered in 1798 that vaccination could prevent smallpox. In 1967, the World Health Organization intensified efforts to eliminate the disease. Smallpox is one of two infectious diseases to have been eradicated, the other being rinderpest in 2011. The term "smallpox" was first used in Britain in the 15th century to distinguish the disease from syphilis, which was then known as the "great pox". Other historical names for the disease include pox, speckled monster, and red plague.
There were two clinical forms of smallpox. Variola major was the severe and most common form, with a more extensive rash and higher fever. Variola minor was a less common presentation, and a much less severe disease, with historical death rates of 1 percent or less. Subclinical (asymptomatic) infections with variola virus were noted but were not common. In addition, a form called variola sine eruptione (smallpox without rash) was seen generally in vaccinated persons. This form was marked by a fever that occurred after the usual incubation period and could be confirmed only by antibody studies or, rarely, by virus isolation.
The incubation period between contraction and the first obvious symptoms of the disease is around 12 days. Once inhaled, variola major virus invades the oropharyngeal (mouth and throat) or the respiratory mucosa, migrates to regional lymph nodes, and begins to multiply. In the initial growth phase the virus seems to move from cell to cell, but around the 12th day, lysis of many infected cells occurs and the virus is found in the bloodstream in large numbers (this is called viremia), and a second wave of multiplication occurs in the spleen, bone marrow, and lymph nodes.