The number of measles cases across the nation just keeps going up, with at least 122 confirmed cases in 18 different states. So far, Massachusetts has not seen any. But experts say there is a risk. 

The whole issue surrounding vaccinations and the fear many people have is the subject of a Nova documentary, called "Vaccines — Calling the Shots."

As a result of health concerns, approximately 10 percent of parents either delay or skip some shots. Around 1 percent don’t vaccinate at all. In some places, those rates are far higher.

Dr. Rick Malley, an infectious disease specialist at Boston Children's Hospital, and Barry Bloom, a scientist and professor at Harvard’s School of Public Health, have done a great deal of research on vaccines and their safety and benefits.