Dr. Paul Offit is the director of the Vaccine Education Center at the Children's Hospital of Philadelphia and a professor of Vaccinology and Pediatrics at the University of Pennsylvania Perelman School of Medicine. He's written tons of published peer-reviewed papers and received countless awards for his work. His latest book, Bad Faith, is all about how religious belief can undermine modern medicine.

On the day of taping, California had just passed a law making vaccinations all but mandatory, Jim Carrey had gone on a Twitter rant about vaccines, and a woman had died of measles.

We spoke with Dr. Offit about why people still don't get vaccinated, how he responds to the most common arguments offered by anti-vaxxers, and why Bill Maher is wrong when he says we don't need to get flu shots.