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.
चेचक के वायरस इंसान के शरीर में रहने से ही ज़िंदा रहते हैं, इसलिए उन्हें मिटाना आसान होता है ।