Health workers vaccinate a child for polio, a highly contagious virus, during the National Immunization Days in Jalalabad, Afghanistan.

Health workers vaccinate a child for polio, a highly contagious virus, during the National Immunization Days in Jalalabad, Afghanistan.

Health workers vaccinate a child for polio, a highly contagious virus, during the National Immunization Days in Jalalabad, Afghanistan, in late April,2014. Alarmed by the spread of polio from conflict zones in three continents, the World Health Organization declared an international health emergency on Monday in an effort to contain the disease, a paralyzing virus that was once thought to be nearly eradicated. (Diego Ibarra Sanchez/The New York Times)