The World Health Organization (WHO) has listed vaccine hesitancy—the delay in acceptance or refusal of vaccines despite their availability—as one of its top 10 health threats facing the world in 2019, according to this story from Newsweek. The phenomenon has taken hold in a number of countries around the world in recent times, and notably in the U.S. as well. Data from the Centers for Disease Control and Prevention shows that while coverage of most recommended vaccines remained relatively stable and high in 2017 for American children aged 19 to 35 months, the percentage who have received no vaccinations has quadrupled since 2001. According to a recent study published in the journal PLOS ONE, increasing numbers of people in many U.S. states now hold anti-vaccination views.