Twelve years ago, the World Health Organization launched the Global Polio Eradication Initiative, with 2000 as its target for eliminating the crippling scourge. At that time, an estimated 350,000 new cases of polio emerged each year. Though the incidence of polio has been dropping dramatically, even in this deadline year, 10 new cases crop up daily somewhere within the 30 countries where the disease remains endemic.
This tally only counts the cases triggered by infection with the virus naturally present in the environment. In addition to this so-called wild-type virus, rivers and sewage host another type—one derived from the very vaccines used to prevent polio. The oral vaccine's live but attenuated virus in rare cases reverts to the disease-causing form. This vaccine-derived virus turns up in natural waters even in regions now certified free of the wild-type virus, argues virologist Hiromu Yoshida of Japan's National Institute of Infectious Diseases in Tokyo.