In California, the number of people diagnosed with full spectrum autism has increased by more than 600 percent over the last 15 years.

Numbers like that have parents and researchers looking desperately for a cause -- even while they disagree over whether the autism explosion is real or simply a factor of better medical care.

Will Medlin is a solitary child -- he doesn't play like a typical 9-year-old.

But it wasn't always that way. As an infant and toddler, he seemed to develop normally until about age 2.

"We went through 5 or 6 day cares rather quickly," said his mom Angela Medlin. "He just couldn't handle being around other kids."

Autism? Or Mercury Poisoning?

The diagnosis for Will was autism. And so went the beginning of his family's quest to figure out why. "The vaccine thing stayed in the back of my mind... had for a long time," Medlin said.

Angela believes Will had a reaction to one of his early baby shots. And after she learned about a lawsuit over thimerosal -- a mercury-based preservative in vaccines -- she researched Will's medical records.

"All the vaccines that he had, had mercury in it and the symptoms he has right now are symptoms that are the same as mercury poisoning," she said.

The Medlins now want Will's official diagnosis changed from autism to mercury poisoning. Every day, he goes through a rigid regimen of diet supplements, and on weekends a process called chelation to try to rid his body of the mercury.

"Finally, I think things are starting to pay off for Will," Angela Medlin said.

Dr. Robert Davis is one of a number of epidemiologists nationwide investigating thimerosal for any link to autism.

"There is no evidence that the amount of thimerosal children get early in life has increased the risk of autism," he said.

The Institute of Medicine says most vaccines no longer contain thimerosal. But those who believe there is a link point to what they call an autism explosion in the 1990's, which is during the same time babies got more shots. But that "explosion" is a matter of debate.

"There may have been the exact same rate of autism 15 years ago and we just missed it," Dr. Davis said.

Cutting Edge Study At UW

Experts at the University of Washington are doing cutting edge research on the causes of autism. They are concentrating on genetics but are keeping an open mind about possible environmental triggers.

"We absolutely don't understand what causes autism at this point," said Dr. Geraldine Dawson.

Dr. Dawson is in the midst of an exhaustive study on causes and treatments for autism. She believes the number of cases may be up because kids with even mild symptoms are now spotted early.

"Now we know autism can be very mildly manifested in a person who is just socially awkward and has trouble communicating in very subtle ways," she said.

Aaron Andersson is part of Dawson's study. He had speech delays at age two, but didn't have some classic autism symptoms.

"For his agem most people think there is something different about him but they don't know what it is," said Shannon Andersson, Aaron's mom.

Both Aaron and his 12-year-old sister are autistic. But doctors missed the diagnosis in Aaron's sister until he joined the UW's research project.

Now he undergoes developmental tests, and a process where special sensors measure the electrical activity in his brain. Dawson already has reported differences in the brains of autistic children.

"At least by age 3, the brain development is quite abnormal and what we are finding is that children with autism have larger than normal brains," said Dawson.

"Hopefully Some Day They'll Know What Causes It'

Dawson hopes studies of the genes that regulate brain development will lead to better early screening and, maybe one day, a cure for autism.

"Hopefully some day they will know what does cause it," Andersson said. "If there's any way to reverse it or ways to prevent it, they need people like my son to figure that out and it just makes me feel good to be a part of that."

Angela Medlin believes she does know what caused her son's developmental disorder: the mercury-based preservative in his baby shots. But she's frustrated the doctors don't agree.

"I have received very little support from the medical community," she said. "I bring it up and I just get a blank stare. They just dismiss it."

Doctors worry that fear of autism could keep people from vaccinating kids. They say that would be a huge mistake they say because shots are the best way to protect children from life-threatening diseases.

Drug companies are making vaccines that don't have the mercury based preservative. Concerned parents should ask their pediatrician to make sure their child's shots are thimerosal free, or ask to see the package insert and check for themselves.

The study on thimerosal and autism should be complete by 2005. And over the next 5 years, experts say we should know if the rate of autism has gone up, or whether the increasing numbers are due to better diagnosing.

For More Information:

University of Washington Autism Center -- depts.washington.edu/uwautism
Families For Effective Autism Treatment of Washington (Also has info on UW Sibling Autism Study) -- www.featwa.org