http://www.who.int/bulletin/volumes/85/07-100107/en/
Q: Why was it easier to eradicate smallpox than polio?
A: Smallpox eradication was a much easier programme because every infection was clinically expressed in the same manner, so you could tell easily when someone had the disease. Also, once you found that a person had smallpox you could isolate them and vaccinate a ring of people around that person. With polio, for every clinically expressed paralysis there are at least 200 children infected asymptomatically, which means you can’t find every polio infection.
It’s amazing. You anti-vaccine people will never answer the question I posed straightforwardly. There is always some issue that prevents you from saying whether you favor or don’t favor children being required to be vaccinated for diseases like polio or measles or whooping cough or rubella.