Eradication and cessation of programmes.

Eradication of disease requires the eradication of the causative agent. Smallpox is the only human disease knowingly eradicated so far and, while poliomyelitis is likely to be the second, they present very different problems. All smallpox infections were apparent when significantly infectious, the vaccine was very easy to deliver and its effects easy to monitor, and it was incapable of causing the disease it was intended to prevent. In contrast, most poliovirus infections are silent, vaccination leaves no outward sign, and there is a low, but real, incidence of vaccine-associated poliomyelitis. While smallpox was eradicated by containment of infections by quarantine and vaccination, poliomyelitis requires mass vaccination campaigns to break virus transmission. Moreover, in contrast to smallpox, the strategies for stopping polio vaccination are still under discussion. The polio eradication programme on the other hand has made and continues to make strides towards its goal, and is a major triumph of public health interventions.