Autism is a developmental disorder characterized by troubles with social interaction and communication and by restricted and repetitive behavior. Parents usually notice signs in the first two or three years of their child's life. These signs often develop gradually, though some children with autism reach their developmental milestones at a normal pace and then worsen. Autism is caused by a combination of genetic and environmental factors. Risk factors include certain infections during pregnancy, such as rubella, as well as valproic acid, alcohol or cocaine use during pregnancy. Controversies surround other proposed environmental causes, for example the vaccine hypotheses, which have been disproven.