Autism or Autism Spectrum Disorder is a neurodevelopment disorder. The "neuro" part of the definition stands for the physiological or tissue variance in the brain. In other words, it may include the difference in the number (↑↓) of brain cells, the growth and location of specific brain function(s), and, finally, difference in brain circuitry. The "developmental" part of the definition stands for the persistent noticeable delays in development. It may also manifest itself in a child missing and/or skipping a certain developmental milestone that later severely affects the overall functioning of that child.
Autism and other Spectrum Disorders are characterized by delays or difficulties (or deficits) in four main areas. These are Social Interaction, Language and Communication, Repetitive/Restricted Behavior (range of activities and interests), and Sensory Processing (auditory, visual, tactical inputs, etc.). In addition to these area of difficulty that are used as diagnostic criteria for ASD, children may experience Anxiety.
Canadian Research Agenda in Early Autism (CAIRN) - Symptoms of Autism :
The range and intensity of the disability varies. Previously, the Autism Spectrum Disorders included Child Disintegrative Disorder (CDD), Rett's Disorder, Autistic Disorder (AD), Pervasive Developmental Disorder - Not Otherwise Specified (PDD-NOS) and Asperger's Syndrome (AS). Please note that based on DSM-V, the new diagnostic manual, we now have one label for the autism continuum, i.e., Autism Spectrum Disorders. Also, CDD has rarely been seen in the population living in North America and is no longer included under ASD. Please review the new diagnostic criteria by visiting the link below.
American Psychiatric Association - DSM-V Diagnostic Criteria
Interacting with Autism, a video-based resource http://www.interactingwithautism.com/
The causes vary from the immune system reaction vaccination(s), environmental causes, quality of nutrition, GI Tract diseases (such as Dysbiosis and others) to genetic predisposition to name a few. The scientific and medical communities are not 100% sure. There is no definite answer, since there are many factors resulting in Autism or ASD in a child.
One of the possible reasons for ASD is brain imbalance. The imbalance hapenes when one hemisphere of the brain matures faster then the other. There is a "Brain Balance Program" developed by Dr. Melillo who claims that it is possible to correct the brain imbalance by proper stimmulation of the weaker part of the brain. Read more about "Brain Balance Program".
Note: Most of the information covered above was learned through some individual reading, education and parent training at Geneva Centre for Autism (2010-2011) and Adventure Place (2011-2012).