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A severe infectious disease caused by a virus, and often fatal. Smallpox caused fever, headache, and general malaise, followed by a rash. The rash characteristically affected face and limbs more than trunk and lesions start simultaneously. From a maculo-papular appearance, the rash passes into a pustular or vesicular stage and ends with scab formation; the lesions are deep and cause scarring. Major and minor forms of smallpox existed, with high fatality rate in major, often with extensive skin hemorrhage. Transmission is from infected cases by secretions and skin lesions; these are infectious for the duration of the rash. Immunization against smallpox was the earliest form practiced, initially through self-inoculation with the minor form. Later Jenner introduced vaccination with the related cowpox virus (later vaccinia were used). In 1980, the World Health Organization declared that the disease had been eliminated. Quarantine and vaccination were crucial to this. Today, smallpox viruses are found only in a few laboratories for research purposes.

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