We've seen Dr. Zimmerman described as many things, including a covert government agent paid to cover up the truth about vaccines.
Unsurprisingly, that's not quite right, and the true story is told so well on Snopes, we recommend giving it a read there.
But here's the gist: Zimmerman is a real doctor. He's a pediatric neurologist who served the government in several one-offs by providing expert testimony against the link between vaccines and autism. Zimmerman is also a single person, not a preponderance of evidence spanning years of studies by a bounty of universities and researchers.
Zimmerman's claim was not that vaccines did cause autism but a statement of curiosity that vaccines "could potentially be a factor that 'stress[ed] mitochondrial reserve."
In 2006, Zimmerman was the co-author of a case study, that is a study of just one subject, the 19-month-old daughter of the other author of the report. Two years later it was revealed the girl was born with a mitochondrial disorder.