Smallpox epidemics once routinely decimated the populations of countries around the world. English historian Thomas B. Macaulay called the disease the "most terrible of all the ministers of death."
Caused by the variola virus, smallpox starts with symptoms such as fever and headache, followed a few days later by a rash that causes severe skin pustules all over the body and scabs that fall off after three or four weeks, sometimes leaving disfiguring pockmarks.
The severe form of smallpox killed 15 percent to 40 percent of its victims; the minor type less than 1 percent.
In 1796, English physician Edward Jenner administered the first vaccination against the disease, which he called "the most dreadful scourge of the human race."
The last documented American outbreak of the minor form of smallpox occurred in the lower Rio Grande Valley in 1949.
But the disease persisted in in Asia, Africa and South America. In 1967, the World Health Organization began a global eradication program.
The last known naturally-acquired case of the most deadly form of smallpox occurred in Bangladesh in 1975.
The last case of the minor form was reported in Somalia in 1977.
Since then, the only two known cases of smallpox occurred in Birmingham, England, in 1978 after a laboratory accident.
In 1979, a global commission certified not only that smallpox was eradicated but that there was no evidence that it would return as a naturally spread disease. The disease is spread only among humans, with no intermediate carriers.
Recommendations to ensure permanent eradication were adopted by a World Health Organization assembly in 1980.
These included discontinuing smallpox vaccination, except for those at special risk of exposure; the establishment of a stockpile of freeze-dried vaccine sufficient to inoculate 200 million people; a WHO system for investigating suspected smallpox cases; and limiting the stock of smallpox virus to no more than two centers and keeping it in special containment facilities to prevent accidental exposure.
According to a 1985 American Public Health Association report, all 164 WHO member states except Albania have discontinued routine civilian vaccination.
In the United States, only military personnel and a small number of laboratory workers who study viruses related to smallpox are still being immunized against the disease.