the modified type, variola sine eruptione, flat type, and hemorrhagic type (Fenner et al., 1988). Before the eradication of the variola virus, ordinary type smallpox accounted for approximately 90 percent of cases in unvaccinated individuals and 70 percent in previously vaccinated individuals whose immunity had weakened over time (CDC, 2002b).

The variola virus spreads relatively slowly (Fenner et al., 1988). Its transmission generally occurs through aerosols or respiratory-droplet nuclei that settle on the nasal or oropharyngeal mucosal membranes or on the alveoli of the lungs, and also (though less frequently) through infected bedding or clothing. The disease is less infectious than measles or influenza, requiring considerable exposure to an infected person, such as that found in the household or in the health care setting (Breman and Henderson, 2002; Henderson et al., 1999). Furthermore, a person infected with smallpox is not infectious during the incubation stage of the disease, which may range from 7 to 17 days (19 days has also been reported), with a mean of 10-12 days (Breman and Henderson, 2002; Fenner et al., 1988; Henderson et al., 1999; IOM, 1999). Although this stage is free of observable symptoms, it is a period of intense viral replication and spread to internal organs. The disease’s prodromal (initial symptoms) stage, which lasts 2 to 4 days, is characterized by the sudden onset of severe headache, backache, and fever, sometimes vomiting, and less frequently, diarrhea (Breman and Henderson, 2002; CDC, 2002a). Individuals in the prodromal stage may be contagious. The prodromal stage is followed by eruption into a rash with lesions on the skin and lesions of the oral mucosa, which shed large amounts of the virus. The early rash stage is followed by the progression of the lesions simultaneously from macules, to papules, which become vesicles, then pustules, and finally, crusts or scabs (CDC, 2002a). Individuals remain infectious, though less so, until the last scab has separated from the skin, 3 to 4 weeks after the onset of the rash (CDC, 2003e).

Smallpox infection leads to a generally distinctive rash. However, smallpox has not been part of the diagnostic experience of most currently practicing health care providers, and smallpox disease could be confused with certain drug reactions and other diseases (such as chickenpox) or skin conditions. The smallpox rash may be distinguished by its centrifugal distribution—lesions are found in greater concentration at the extremities, on the face, hands, and feet, but as the disease progresses, they generally cover the entire body—and the fact that all pustules in a given area develop and progress at the same time rather than in crops (Fenner et al., 1988). Definitive diagnosis can be confirmed in the laboratory; the shape of the variola virus is different from that of varicella-zoster, the cause of chicken pox, and a polymerase-chain-reaction assay is the definitive method for identifying variola virus (Breman and Henderson, 2002).