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Where Does Smallpox Come From

Smallpox is caused by exposure to the variola virus. The variola virus emerged in human populations thousands of years ago. Except for laboratory stockpiles, the variola virus has been eliminated. However, in the aftermath of the events of September and October, 2001, there is heightened concern that the variola virus might be used as an agent of bioterrorism. For this reason, the U.S. government is taking precautions for dealing with a smallpox outbreak. 1

Smallpox Transmission

Generally, transmission of smallpox from one person to another occurs through inhalation of airborne variola virus through direct contact with infected bodily fluids (such as droplets expressed from the oral, nasal, or pharyngeal mucosa of an infected person). It can also occur through contaminated objects such as bedding or clothing. Rarely, smallpox has been spread by virus carried in the air in enclosed settings such as buildings, buses, and trains. 2 The virus can cross the placenta, but the incidence of congenital smallpox is relatively low.5 The Smallpox virus can be transmitted throughout the course of the illness, but transmission is most frequent during the first week of the rash, when most of the skin lesions are intact.3 At this stage the infected person is usually very sick and not able to move around in the community. Infectivity wanes in 7 to 10 days when scabs form over the lesions, but the infected person is contagious until the last smallpox scab falls off.4

Smallpox is highly contagious, but generally spreads more slowly and less widely than some other viral diseases, perhaps because transmission requires close contact and occurs after the onset of the rash. The overall rate of infection is also affected by the short duration of the infectious stage. In temperate areas, the number of smallpox infections were highest during the winter and spring. In tropical areas, seasonal variation was less evident and the disease was present throughout the year.3 Age distribution of smallpox infections depends on acquired immunity. Vaccination immunity declines over time and is probably lost in all but the most recently vaccinated populations.5 Smallpox is not known to be transmitted by insects or animals and there is no asymptomatic carrier state.3

References:
1.Ryan KJ, Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. pp. 525–8. ISBN 0838585299.
2.Behbehani AM (01 Dec 1983). “The smallpox story: life and death of an old disease”. Microbiol Rev 47 (4): 455–509. PMID 6319980. PMC 281588. http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=6319980.
3.Barquet N, Domingo P (15 Oct 1997). “Smallpox: the triumph over the most terrible of the ministers of death”. Ann. Intern. Med. 127 (8 Pt 1): 635–42. PMID 9341063. http://www.annals.org/cgi/content/full/127/8_Part_1/635.
4.Behbehani AM (01 Dec 1983). “The smallpox story: life and death of an old disease”. Microbiol Rev 47 (4): 455–509. PMID 6319980. PMC 281588. http://mmbr.asm.org/cgi/pmidlookup?view=long&pmid=6319980.
5.”Smallpox”. Armed Forces Institute of Pathology: Department of Infectious and Parasitic Diseases. http://web.archive.org/web/20071009141639/ and http://www.afip.org/Departments/infectious/sp/text/1_1.htm. Retrieved 2009-10-28.

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