KANDAHAR, Afghanistan — At just 2 years old, Madina had been vaccinated seven times. It was not enough: When she fell ill this fall, a trip to the doctor in Kandahar City confirmed that she was among the latest Afghan toddlers to contract polio.

Almost a million children in Kandahar Province alone, like Madina, need at least one dose of oral vaccine a month to head off the disease, health workers say. But many of them also live in the most violent and socially disrupted parts of southern Afghanistan, where the Taliban control large areas and do not want government health workers going door to door.

These realities make a sustained vaccination campaign brutally difficult for health workers here. And Afghanistan, one of three countries where polio is still endemic, is losing ground. Officials have registered 19 cases of polio so far this year, up from 13 each of the previous two years, according to World Health Organization figures.

In August, I followed a polio vaccination crew in Kandahar and saw how, even in areas where health workers have been able to function, many things stand in their way: security fears and drought, deep poverty and stifling tradition, widespread illiteracy and superstition.