Although smallpox has been eliminated as a naturally-occurring disease, the virus still exists in two approved laboratories in the United States and Russia. The Centers for Disease Control and Prevention classifies it as a "Category A" agent, presenting the greatest potential threat for harming public health if developed and used as a bioterrorist agent.
Smallpox is caused by the variola virus, which emerged thousands of years ago. Variola major, the most common form of the virus, is divided into four subcategories: ordinary (which accounts for about 90 percent of cases and has a fatality rate of about 30 percent), modified (which occurs in people who have been vaccinated and has a death rate of about 10 percent), and flat and hemorrhagic (both very rare and uniformly fatal).
According to the CDC, exposure to the variola virus is followed by an incubation period of seven to 17 days, during which people are not contagious and feel fine. The first symptoms emerge during what is called the prodrome phase, and they include head and body aches, fatigue, a high fever and, sometimes, vomiting. This phase lasts two to four days and at this point people may or may not be contagious. Then a rash emerges all over the body and grows increasingly severe over the next 20 or so days, eventually forming scabs; during this period people are contagious, particularly during the first seven to 10 days of the rash. The disease eventually resolves and contagion ends after all of the scabs have fallen off. People who survive are then considered to be immune from smallpox.
A person can become infected by prolonged, face-to-face exposure with someone who is contagious, direct contact with infected bodily fluids or a contaminated object, such as bedding or clothing, and exposure to an aerosol release.
Routine smallpox vaccination ended in 1972, which leaves at least
Source:Fred Hutchinson Cancer Research Center