"[W]hooping cough is resurgent: just under fifty thousand people caught it in 2012. This is mainly because protection from the current shots wanes over time. But, as the bug circulates, it is also morphing. Last February, researchers identified mutant strains of the bacteria in eleven patients in Philadelphia. These mutants, it turns out, appear to be the dominant form of the bug in this country. Whooping cough has evolved in a way that vaccine-preventable diseases rarely do."