Smallpox is a virus that causes a rash that then turns into painful blisters. the last known case of smallpox was in 1977. what is the best explanation for the eradication of smallpox?

It has been recorded that the last case of small pox occurred in 1977 and since then no other case has been seen.  

According to World Health Organization, small pox has been eradicated. The best justification is the occurrence of ring vaccination. It is basically a strategy to control the occurrence of any small pox case by providing vaccination to people who are at the verge to develop it. Ring vaccination justifies that there is no small pox case occurred since then.

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