Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder that affects communication and behavior. It is called a "spectrum" disorder because people with ASD can have a range of symptoms and varying levels of impairment. The exact cause of ASD is not fully understood, but research suggests that both genetic and environmental factors may play a role.

There are several subtypes of ASD, including:
1. Autistic disorder: This is the most severe form of ASD and is characterized by significant impairments in communication and social skills, as well as repetitive behaviors and restricted interests.
2. Asperger syndrome: This is a milder form of ASD that is characterized by difficulties with social interactions and communication, but with no significant delay in language development or cognitive functioning.
3. Pervasive Developmental Disorder-Not Otherwise Specified (PDD-NOS): This is a subtype of ASD that is characterized by some symptoms of ASD, but not enough to meet the criteria for one of the other subtypes.

The exact cause of ASD is not known, but research suggests that both genetic and environmental factors may play a role. In some cases, ASD may be caused by a combination of genetic and environmental factors.

Genetic factors: ASD tends to run in families, and studies have identified a number of genes that may be associated with the disorder.
In some cases, ASD can be caused by a genetic mutation or chromosomal abnormality.

Environmental factors: Exposure to certain chemicals or viruses during pregnancy may increase the risk of ASD.
Complications during birth, such as a lack of oxygen, may also increase the risk of ASD.

It is important to note that the majority of cases of ASD do not have a clear cause. Further research is needed to better understand the causes of ASD and to develop effective treatments.