Autism spectrum disorder (ASD) is a developmental disability caused by differences in the brain. Scientists believe that there are multiple causes of ASD that act together to change the most common ways people develop. While there is no single known cause of ASD, research suggests that autism develops from a combination of genetic and nongenetic, or environmental, influences. Several different genes appear to be involved in autism spectrum disorder. Additionally, environmental factors such as an infection or contact with chemicals in the environment could cause autism in someone who is susceptible because of genetic factors. However, because the disorder is so complex and no two people with autism are exactly alike, there are probably many causes for autism. Scientists are still studying the possible causes of or contributors to ASD.