One Child at a Time

The stone houses of Safal Bandi in Pakistan's remote Swat Valley have changed little over the past century. The only indication of the passage of time is a series of dates, six weeks apart, chalked on the door of each house. Parwanna Begum steps from behind one of those doors and adds today's date to the list, followed by a fraction: 2/3. Out of three children under the age of five in the household, Begum, a local health worker, has vaccinated two against polio. The third is out playing. Begum will have to come back tomorrow, because one child missed in...