Eradication of smallpox is the best known example of WHO’s accomplishments. Indeed, the benefits of smallpox eradication for public health are very impressive. In 1967, when WHO started international eradication efforts, smallpox, was estimated to have afflicted up to 15 million people annually, of whom some two million died with millions more left disfigured and sometimes blind. With the amazing global cooperation, in 1980, WHO was able to certify that the disease had been eradicated.

If smallpox were not eradicated, there would have been 350 million new victims in the past 20 years -- roughly the combined population of the USA and Mexico -- and an estimated 40 million deaths -- a figure equal to the entire population of Spain or South Africa.(WHO http://www.who.int/archives/who50/en/smallpox.htm)

Pictures taken from lectures Modeling Potential Response to Smallpox by Martin Meltzer, CDC, USA http://www.inta.cl/supercourse/lecture/cdc0111/003.htm

And

Microbial threats to health in the US http://www.pitt.edu/~super1/lecture/lec6991/index.htm by Joshua Lederberg