An anonymous reader writes with this extract from a Reuters article: "In early 2010, a spike in cases appeared at Kaiser Permanente in San Rafael, and it was soon determined to be an outbreak of whooping cough — the largest seen in California in more than 50 years. Witt had expected to see the illnesses center around unvaccinated kids, knowing they are more vulnerable to the disease. 'We started dissecting the data. What was very surprising was the majority of cases were in fully vaccinated children. That's what started catching our attention,' said Witt."