The number of autism cases in the United States continues to rise.

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Some children seem to live in a world all their own. They appear distant and closed-in, avoiding eye contact and shying away from their parents' hugs. They may exhibit unusual behaviors, like flapping their arms or obsessively lining up their toys. They are among the approximately 1.5 million children in America who have autism -- a condition that interferes with a child's ability to communicate and interact socially.

Autism has up to now been very difficult to decipher. Scientists still don't fully understand the mystery that lies deep in the brains of children with autism. Although researchers have uncovered clues to what causes the condition, they have not yet discovered a way to prevent or cure autism.

­What is concerning to many scientists is that autism appears to be on the rise. Today, one out of every 110 American children has the condition, according to the Centers for Disease Control and Prevention (CDC). Since the 1980s and early 1990s, the rate of autism has increased by approximately 10 to 17 percent annually. Doctors don't know whether this apparent increase is the result of some environmental factor -- such as greater exposure to toxic chemicals -- or simply that children are being diagnosed more effectively than they were two decades ago.  On the next page, we'll take a closer look at the definition and characteristics of autism.