Jordan King was a typical baby. His parents called him vocal and vivacious. Then just before age 2, after a large battery of vaccinations, he simply withdrew from the world. "The real scary thing was when I noticed he wasn't looking at us any more in the eyes," Mylinda King, Jordan's mother, said. William Mead was a Pottery Barn baby model and met all the typical milestones. Then, also at age 2, after a set of vaccinations, William became very ill and he, too, changed forever. At first, both sets of parents suspected hearing problems.

"The reason we had him tested for a hearing deficit was 'cause he wouldn't respond to us," Mead said. "He no longer used any of his language. We had him tested for deafness, it was that bad," King said. "I mean, you could slam a book on the floor and he wouldn't turn around to see what the sound was. It was like he was in this bubble of somewhere else, like he'd left the planet or something."

Doctors said it wasn’t a hearing problem … it was the brain disorder autism.

In both children, batteries of tests revealed dangerous levels of the brain toxin mercury in their systems. Their only known exposure: the mercury preservative once widely used in childhood shots. "Our doctor, Dr. Green, said 'you can stop looking for sources'," King said. "I know where it came from and it was … when he told us it was the vaccines, you just can't believe it."

Now, William and Jordan are two test cases among nearly 5,000 autism claims filed in federal vaccine court. Most claim that mercury, or MMR shots, or both, resulted in their children’s autism. Government officials and many scientists insist there’s nothing about vaccines that can lead to autism.

"I think it's important for the average parent to know that the government hasn't made a link between vaccines and autism," said Dr. Anne Schuchat of the Centers for Disease Control. Dr. Bernadine Healy is the former head of the National Institutes of Health, and the most well-known medical voice yet to break with her colleagues on the vaccine-autism question. In an exclusive interview with Sharyl Attkisson of CBS News, Healy said the question is still open.

"I think that the public health officials have been too quick to dismiss the hypothesis as irrational," Healy said. "But public health officials have been saying they know, they've been implying to the public there's enough evidence and they know it's not causal," Attkisson said.

"I think you can't say that," Healy said. "You can't say that."

Healy said: "There is a completely expressed concern that they don't want to pursue a hypothesis because that hypothesis could be damaging to the public health community at large by scaring people. "First of all," Healy said, "I think the public’s smarter than that. The public values vaccines. But more importantly, I don’t think you should ever turn your back on any scientific hypothesis because you’re afraid of what it might show."