223 -- That's how many polio cases there were in the world last year, an extraordinary drop from 350,000 in 1988. That tremendous progress is the result of a multi-decade, multi-billion-dollar global effort to eradicate the disease by 2018. But there has been an outbreak in Somalia this year, with 121 cases of the infamous crippler of children reported in a region controlled by a rebel group resistant to the global vaccination campaign. The outbreak shows just how difficult it is to wipe out a disease -- and how important it is to eliminate every last case. That's the only way to make sure the scourge won't make a comeback.