From Demon in the Freezer
...Smallpox is explosively contagious, and it travels through the air. Virus particles in the mouth become airborne when the host talks. If you inhale a single particle of smallpox, you can come down with the disease. After you've been infected, there is a typical incubation period of ten days. During that time, you feel normal. Then the illness hits with a spike of fever, a backache, and vomiting, and a bit later tiny red spots appear all over the body. The spots turn into blisters, called pustules, and the pustules enlarge, filling with pressurized opalescent pus. The eruption of pustules is sometimes called the splitting of the dermis. The skin doesn't break, but splits horizontally, tearing away from its underlayers. The pustules become hard, bloated sacs the size of peas, encasing the body with pus, and the skin resembles a cobbled stone street.
The pain of the splitting is extraordinary. People lose the ability to speak, and their eyes can squeeze shut with pustules, but they remain alert. Death comes with a breathing arrest or a heart attack or shock or an immune-system storm, though exactly how smallpox kills a person is not known. There are many mysteries about the smallpox virus. Since the seventeenth century, doctors have understood that if the pustules merge into sheets across the body the victim will usually die: the virus has split the whole skin. If the victim survives, the pustules turn into scabs and fall off, leaving scars. This is known as ordinary smallpox.
Some people develop extreme smallpox, which is loosely called black pox. Doctors separate black pox into two forms -- flat smallpox and hemorrhagic smallpox. In a case of flat smallpox, the skin remains smooth and doesn't pustulate, but it darkens until it looks charred, and it can slip off the body in sheets. In hemorrhagic smallpox, black, unclotted blood oozes or runs from the mouth and other body orifices. Black pox is close to a hundred per cent fatal. If any sign of it appears in the body, the victim will almost certainly die. In the bloody cases, the virus destroys the linings of the throat, the stomach, the intestines, the rectum, and the vagina, and these membranes disintegrate. Fatal smallpox can destroy the body's entire skin -- both the exterior skin and the interior skin that lines the passages of the body.
Smallpox virus's scientific name is variola. It means "spotted" in Latin, and it was given to the disease by a medieval bishop. The virus, as a life form, comes in two subspecies: Variola minor and Variola major. Minor is a weak mutant, and was first described in 1863 by doctors in Jamaica. People usually survive it. Classic major kills one out of three people if they haven't been vaccinated or if they've lost their immunity. The death rate with major can go higher -- how much higher no one knows. Variola major killed half of its victims in an outbreak in Canada in 1924, and presumably many of them developed black pox. Smallpox is less contagious than measles but more contagious than mumps. It tends to go around until it has infected nearly everyone.
Most people today have no immunity to smallpox. The vaccine begins to wear off in many people after ten years. Mass vaccination for smallpox came to a worldwide halt around twenty-five years ago. There is now very little smallpox vaccine on hand in the United States or anywhere else in the world. The World Health Organization once had ten million doses of the vaccine in storage in Geneva, Switzerland, but in 1990 an advisory committee recommended that most of it be destroyed, feeling that smallpox was longer a threat. Nine and a half million doses are assumed to have been cooked in an oven, leaving the W.H.O. with a total supply of half a million doses -- one dose of smallpox vaccine for every twelve thousand people on earth. A recent survey by the W.H.O. revealed that there is only one factory in the world that has recently made even a small quantity of the vaccine, and there may be no factory capable of making sizable amounts. The vaccine was discovered in the age of Thomas Jefferson, and making a lot of it would seem simple, but so far the United States government has been unable to get any made at all. Variola virus is now classified as a Biosafety Level 4 hot agent -- the most dangerous kind of virus -- because it is lethal, airborne, and highly contagious, and is now exotic to the human species, and there is not enough vaccine to stop an outbreak. Experts feel that the appearance of a single case of smallpox anywhere on earth would be a global medical emergency...
"Modeling Potential Responses to Smallpox as a Bioterrorist Weapon" (see Table 1 if you like creepy statistics)