As measles cases in the U.S. reach their highest level in a quarter century and RFK Jr. leads an anti-vaccine push from the federal government, it's crucial to understand the importance of vaccines. Historian and University of Oklahoma professor Kyle Harper tells Ali Velshi that "vaccines are arguably the innovation, the discovery that has done more than any other to contribute to human health, to longevity, and to overall human wellbeing," and that "for most of the human past, infectious diseases were difficult or impossible to control." For more context and news coverage of the most important stories of our day click here: msnbc.com » Subscribe to MSNBC: youtube.com/msnbc » Subscribe to MSNBC on TikTok tiktok.com/@msnbc » Subscribe to MSNBC on Instagram instagram.com/msnbc Download our new MSNBC app for the latest breaking news and daily headlines at a glance: msnbc.com/information/download-msnbc-app-n1241692 Follow MSN...