evidence: Gerber JS, Offit PA. Vaccines and autism: a tale of shifting hypotheses. Clin Infect Dis. 2009;48(4):456–461. [...] Bob: This article reviews the three most commonly proposed hypotheses for vaccine-induced development of autism: (1) the MMR vaccine damages the intestinal lining, allowing the entrance of encephalopathic proteins; (2) thimerosal induces central nervous system toxicity; and (3) multiple vaccinations overwhelm and weaken the immune system. This article looks at the genesis of each theory and the data that debunk them.2 [...] Bob: Let's briefly look at the second hypothesis of thimerosal-induced neurotoxicity. Thimerosal is an antibacterial agent that has been used in multidose vaccine preparations for more than 50 years. It is 50 percent ethyl mercury by weight. However, mercury poisoning has a distinctly different presentation than autism. The CDC has also demonstrated that the mercury in vaccines has not resulted in any subtle signs or symptoms of mercury poisoning.10 The authors of this month's article review seven large-scale studies—again, ecologic, retrospective, and prospective studies—all demonstrating no association between thimerosal and autism.2