Please Explain: Polio and Why It's on the Rise

A Pakistani health worker administers a polio vaccination to a child during a polio immunisation campaign in Peshawar on February 2, 2014.

This week, for the first time ever, the World Health Organization declared the spread of polio an international public health emergency that undermines the nearly 30-year effort to eradicate disease. Dr. Rebecca Martin, Director of the Global Immunization Division in the Center for Global Health at the Centers for Disease Control and Prevention, talks about the crippling disease polio, the global effort to eradicate it, and why there’s been an increase in polio infections this year in Pakistan, and a re-emergence in countries previously free of the disease, such as Syria, Somalia and Iraq.