Vaccines and Herd Immunity, An Explainer

Measles was eradicated in the U.S. in 2000, but there are at least 102 reported cases of the disease this year. 

As the measles outbreak that started in Southern California continues to spread, Stephen Morse, epidemiologist from the Mailman School of Public Health at Columbia University, explains why herd immunity is so necessary to prevent a serious outbreak, what the anti-vaccine movement has to do with it, and takes your questions. Plus: why private schools could be an engine for risk.