Reports from Southern California over the weekend suggest that at least some of the people who are against vaccinating their children against measles have changed their minds. People living in areas where an unusually high proportion of the population are not vaccinated are rushing to doctors’ offices and clinics and demanding vaccine for their kids – even though they were staunch anti-vaxxers in the past. One big reason for this is that in these districts, because so many people aren’t vaccinated, the concept of herd immunity doesn’t apply.

How does herd immunity work? Marketplace senior editor and resident explainer Paddy Hirsch explains the theory of herd immunity in less than two minutes in this colorful, amusing video.