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The birthday-party invitation said “siblings welcome,” which means you can bring your 11-month-old son while your husband is out of town. You arrive a little disheveled and a little late. Your 5-year-old daughter rushes into the living room, and you make your way to the kitchen, wearing your son in a sling. You find a few moms around a table arrayed with plates of fruit, hummus, celery sticks, and carrots—no gluten, no nuts, no Red 40. These parents care about avoiding pesticides, screen time, and processed foods, and you do too.
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.
