It’s a classic kids’ party: Tears and lemonade are spilled; mud and cake get smeared into the rug; confetti balloons are popped one by one, showering elated children in rainbow-paper flakes. Sunbeams through the windows illuminate floating dust motes—and, imperceptibly, microdroplets of mucus carrying the measles virus, expelled from an infected but asymptomatic child who is hopping and laughing among the others. Your daughter breathes that same air, inhaling the virus directly into her respiratory tract.