This year, more than 24,000 children will be diagnosed with autism.

Autism is a complex neurological disorder and is the most common condition in a group of developmental disorders called autism spectrum disorders (ASDs).

About one in 150 children is diagnosed with autism; it is more common than pediatric cancer, diabetes and AIDS combined. It occurs in all ethnic and social groups and is four-times more likely to occur in boys than girls. Experts estimate three to six children out of every 1,000 will be diagnosed with autism.

Autism is a puzzling disorder. Scientists don't know what causes it, but it's believed that both environment and genetic factors play a role.

A number of genes associated with autism have now been identified, and studies of people with autism have revealed irregularities in several regions of the brain. Other research suggests that autism could be the result of disruption of normal brain development early in fetal development, possibly caused by defects in the genes that control the growth of the brain and how neurons communicate with one another. These findings are very preliminary, and the studies continue.

Autistic children tend to have less sensitivity to pain but a heightened sensitivity to sound, touch and other sensory stimulation. Children diagnosed with autism appear to have a higher than normal risk of specific medical conditions, including Fragile X syndrome, which causes mental retardation, learning disabilities, epileptic seizures, Tourette syndrome, and tuberous sclerosis (in which tumors develop on the brain). Research still has not determined why, but between 20 percent and 30 percent of children with autism develop epilepsy by the time they become adults.

New guidelines from the American Academy of Pediatrics recommend that every child between 18 months and 24 months be screened for autism. It's not an easy diagnosis. Because autism varies widely in its severity and its symptoms, it can often go undiagnosed, or diagnosis can be delayed.

Three distinctive behaviors characterize autism:

Difficulties with social interaction

Problems with verbal and non-verbal communication

Repetitive behaviors or narrow, obsessive interests

Because autism is a complex disorder, diagnosis is a multi-disciplinary process, involving a team of neurological, behavioral, speech and other professionals in the evaluation process. Often, it's the parents who are first to notice the signs of something "different" in the child's behavior, and this can be the key to early intervention, which can be crucial to effective treatment.

To help parents see potential red flags for autism in their young children, researchers at Florida State University have teamed up with two autism advocacy organizations Autism Speaks and First Signs to develop an online autism glossary. The web tool, at autismspeaks.org, includes more than 100 video clips, showing normal childhood behavior and behaviors that are a red flag for autism.

Autism researcher Amy Wetherby, Ph.D., developed the glossary. She said it is not designed to be a diagnostic tool, but rather an educational tool for parents, "to help them understand behaviors their child may be showing and know whether these are behaviors that they need to be worried about and that they need to seek professional information about."