Smallpox is "easy" to get rid of because it's visible. The infected present with lesions. It also isn't contagious while it incubates. Contrast this with polio

Smallpox is "easy" to get rid of because it's visible. The infected present with lesions. It also isn't contagious while it incubates. Contrast this with polio, where there are about a thousand infections per every paralysis. Thanks to a couple of ignorant leaders who claimed that the polio vaccine is a Western conspiracy to sterilize African girls, it's spreading again. Great.