By the time the dust kicked up in that meeting had settled, Halsey would be forced to reckon with the hypothesis that thimerosal had damaged the brains of immunized infants and may have contributed to the unexplained explosion in the number of cases of autism being diagnosed in children.

That Halsey was willing even to entertain this possibility enraged some of his fellow vaccinologists, who couldn't fathom how a doctor who had spent so much energy dismantling the arguments of people who attacked vaccines could now be changing sides. But to Halsey's mind, his actions were perfectly consistent: he was simply working from the data. And the numbers deeply troubled him. ''From the beginning, I saw thimerosal as something different,'' he says. ''It was the first strong evidence of a causal association with neurological impairment. I was very concerned.''