Yet another outbreak of a vaccine-preventable disease — this time measles in the Pacific Northwest — provides another reminder of the importance of getting children fully vaccinated, on schedule. Anti-vax or vaccine-skeptical parents are certainly not acting out of malevolence. They’re doing what they believe will protect their children’s health. But the fact is they’re achieving the opposite — potentially endangering their health. Those are the facts. Watch the video with TIME’s Editor-at-Large Jeffrey Kluger above to learn why.