The Exit Strategy.

If the assault on polio's remaining strongholds is successful, poliovirus will no longer be circulating. When the polio eradication program was hatched in 1988, it sounded straightforward enough, halt transmission of the wild virus, probably destroy cultures of the virus, and then stop vaccinating. Other tough issues include how to be sure the virus is really gone. For smallpox, the only disease eradicated to date, surveillance was relatively simple, infection causes a visible rash that even an untrained villager could detect. By contrast, polio can circulate invisibly without causing disease or symptoms, diagnosed only with a lab analysis.