Smallpox was the ideal disease to combat by an international vaccination campaign because, unlike diseases that are spread by troublesome vectors such as rats and insects, the smallpox virus depends on a human host for its survival.
कारण उंदीर - घुशी व कीटकांसारख्या त्रासदायक वाहकांद्वारे फैलावणाऱ्‍या रोगांपेक्षा हा वेगळा होता.