Researchers and public health officials have struggled to explain the resurgence of whooping cough in the United States since the late 1970s, and the suspected shortcomings of the current generation of vaccines are often blamed. But a new University of Michigan-led study concludes that the resurgence of the highly contagious respiratory disease is the result of factors–including a phenomenon known as the honeymoon period–that began in the middle of the last century, long before the latest vacc…