In the meantime, behavioral scientists are trying to identify the early symptoms so that a diagnosis can be made by the age of one. As correspondent Lesley Stahl reports, today most children are left undiagnosed until they're five years old.
Researchers at the M.I.N.D. Institute at the University of California in Davis believe, if they can catch it early, they can change the way a child's brain develops. They have started testing their theory in toddlers like Christian Heavin.
Psychologist Sally Rogers, a pioneer in the field of autism treatment, started giving three-year-old Christian intensive therapy about a year ago, hoping to alter the course of his autistic behavior.
Asked what his behavior was like before she met him, Rogers says, "Well, when we first met Christian he didn't have any words."
"He didn't really have any play skills. He mostly threw things on the floor," she adds.
And she says he would throw 20-minute temper tantrums because he couldn't communicate. "He was really out of control," Rogers says. "They had to bolt the furniture to the walls because this two year old was in danger of pulling furniture down on himself."
Dr. Rogers worked with Christian one on one—on her hands and knees, in his face, teaching him new words and forcing him to interact with her.
She believes that if treatment can begin this early, while a child's brain is still malleable, the results can be dramatic.
"Do you think that you're actually re-wiring the brain? Do you think you're setting up new wires that wouldn't be there?" Stahl asks.
"I think we certainly are creating new connections in the brain. That's what learning is," Rogers explains.
Asked if she is suggesting that autism can be cured, Rogers says, "We don't know how to touch the biology of autism. But I do think that the behaviors that are associated with autism can be reduced to the point where they're not obvious anymore."
"Now, you can't make that promise to everybody, can you?" Stahl asks.
"No, you sure can't. There's a huge range of severity in autism. There's a huge range of reactions to treatments," Rogers acknowledges.
Christian is now able to talk with his mother Jennifer, and even a stranger like Stahl, in multiple word sentences.
Valerie Arias often wonders what her 13-year-old son Teddy's life would be like if his autism had been treated earlier.
"When Teddy was about six months old, I had him in his car seat, and he just kept flailing his arm over his head," she remembers. "My mother looked at him and she was like, 'Val, I think Teddy has autism.' At six months old, my mother told me that my son had autism. And I said, 'No, he doesn't. There's nothing wrong with my baby.'"
"I was very angry at my mother," she adds. "I didn't speak to her probably for about a year."
What her mother saw was that Teddy never babbled as a baby—he just screamed and grew increasingly violent.
Valerie may have been in denial, but even doctors didn't diagnose Teddy's autism until he was four years old.
By that time, Michael, who is now nine, had been born. In all, she and her husband Aaron have four children, including Paige, 14, and one-year-old Haydn.
Right after Haydn was born, Valerie heard about a study at the M.I.N.D. Institute on early detection of autism. It was focusing on so-called "baby sibs," children like Haydn with an older autistic sibling. So she signed him up.
"Did you know at that point that autism did run in families, does run in families?" Stahl asks.
"I knew that the chances of having another child with autism were greater," Valerie tells Stahl. "But, I figured since Michael didn't have it that everything was okay."