In 1974, a young girl with measles went to school in upstate New York. Even though 97 percent of her fellow students had been vaccinated, 28 ended up contracting the disease. The infected students were spread out across 14 classrooms, but the young girl, the index patient, spent time only in her own classroom. The culprit? A ventilation system operating in recirculating mode that sucked in the viral particles from her classroom and spread them around the school.