The truth is we don't really know. Autism is a diagnosis that describes a collection of symptoms; what causes them is not completely understood. More and more, scientists use the term Autism Spectrum Disorder (ASD), a reflexion of the fact that when we talk about "autism" we are probably looking at "autisms," a number of distinct conditions that share some common traits. This is why there is such a wide range in severity and children with "autism" can be so different. The autism spectrum includes autistic disorder, Rett syndrome, childhood disintegrative disorder, pervasive developmental disorder-not otherwise specified (PDD-NOS) and Asperger syndrome. Individuals with these diagnoses have challenges related to social interaction, verbal and nonverbal communication and repetitive behaviors. So many kids on the spectrum also have sensory issues and/or gastrointestinal distress that many in the autism community feel these symptoms should also be considered in the diagnosis, but they currently are not.
In the autism community, we use the puzzle piece as a symbol of the puzzle that is autism, and the pieces we must put together to find a cure or at least effective treatments. But if we are really looking at a bunch of different "autisms," it's like having 5 or 10 different puzzles. Until scientists figure out the defining features of the different autisms, it's like we are trying to put together a single puzzle using the pieces from several . No wonder it's a tricky puzzle to solve!
As a parent I've always seen autism not as a puzzle, but an onion. It's layers are so fascinating, but they can definitely make you cry. As my child grows, I peel back the layers. First, the causal layers: genetic predisposition, enviornmental exposures, immune system dysfunction. I try to understand and treat the symptoms, peeling more layers: behavioral therapy (ABA), biomedical treatments, immune system treatments, sensory integration therapy. All the while, the child inside is the same, pure and precious, unique and perfect, not more or less than any other.