Autism is a complex developmental disorder that appears in the first 3 years of life, although it is sometimes diagnosed much later. It affects the brain's normal development of social and communication skills.

Autism is a spectrum that encompasses a wide range of behavior. The common features include impaired social interactions, impaired verbal and nonverbal communication, and restricted and repetitive patterns of behavior.

The symptoms may vary from moderate to severe. Two related, milder conditions are Asperger syndrome and "pervasive development disorder not otherwise specified" (PDD-NOS).

Autism is a physical condition linked to abnormal biology and chemistry in the brain. The exact causes of these abnormalities remain unknown, but this is a very active area of research. There are probably a combination of factors that lead to autism.

Genetic factors seem to be important. For example, identical twins are much more likely than fraternal twins or siblings to both have autism. Similarly, language abnormalities are more common in relatives of autistic children. Chromosomal abnormalities and other neurological problems are also more common in families with autism.

A number of other possible causes have been suspected, but not proven.  They involve digestive tract changes, diet, mercury poisoning, vaccine sensitivity, and the body's inefficient use of vitamins and minerals.

The exact number of children with Autism is not known, but estimates suggest that roughly 1 in 1,000 children are affected. Autism affects boys 3 to 4 times more often than girls. Family income, education, and lifestyle do not seem to affect the risk of autism.

Some parents have heard that the MMR vaccine that children receive may cause autism. This theory was based, in part, on two facts. First, the incidence of autism has increased steadily since around the same time the MMR vaccine was introduced. Second, children with the regressive form of autism (a type of autism that develops after a period of normal development) tend to start to show symptoms around the time the MMR vaccine is given. This is likely a coincidence due to the age of children at the time they receive this vaccine.

Several major studies have found NO connection between the vaccine and autism, however. The American Academy of Pediatrics and the Center for Disease Control and Prevention report that there is no proven link between autism and the MMR vaccine.

Some doctors attribute the increased incidence in autism to newer definitions of autism. The term "autism" now includes a wider spectrum of children. For example, a child who is diagnosed with high-functioning autism today may have been thought to simply be odd or strange 30 years ago.