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Different underlying brain dysfunctions have been hypothesized to result in the common symptoms of autism, just as completely different brain types result in [[intellectual disability]].<ref name="Waye_2018" /><ref name="Hodges_2020">{{cite journal | vauthors = Hodges H, Fealko C, Soares N | title = Autism spectrum disorder: definition, epidemiology, causes, and clinical evaluation | journal = Translational Pediatrics | volume = 9 | issue = Suppl 1 | pages = S55–S65 | date = February 2020 | pmid = 32206584 | pmc = 7082249 | doi = 10.21037/tp.2019.09.09 | doi-access = free }}</ref> In recent years, the prevalence and number of people diagnosed with the disorder have increased dramatically. There are many potential reasons for this occurrence, particularly the changes in the diagnostic criteria for autism.<ref name="Salari_2022">{{cite journal | vauthors = Salari N, Rasoulpoor S, Rasoulpoor S, Shohaimi S, Jafarpour S, Abdoli N, Khaledi-Paveh B, Mohammadi M | display-authors = 6 | title = The global prevalence of autism spectrum disorder: a comprehensive systematic review and meta-analysis | journal = Italian Journal of Pediatrics | volume = 48 | issue = 1 | pages = 112 | date = July 2022 | pmid = 35804408 | pmc = 9270782 | doi = 10.1186/s13052-022-01310-w | doi-access = free }}</ref>
Environmental factors that have been claimed to contribute to autism or exacerbate its symptoms, or that may be important to consider in future research, include certain foods,<ref name="Quan_2019">{{Cite journal | vauthors = Quan J, Panaccione N, King JA, Underwood F, Windsor JW, Coward S, Gidrewicz D, Kaplan GG | display-authors = 6 |title=A257 Association Between Celiac Disease and Autism Spectrum Disorder: A Systematic Review |date=March 2019 |journal=Journal of the Canadian Association of Gastroenterology |volume=2 |issue=Supplement_2 |pages=502–503 |doi=10.1093/jcag/gwz006.256 |issn=2515-2084 |pmc=6512700}}</ref> [[infectious disease]], [[heavy metals]], [[Solvent|solvents]], [[diesel exhaust]], [[Polychlorinated biphenyl|PCBs]], [[phthalates]] and [[Phenol|phenols]] used in [[plastic]] products, [[Pesticide|pesticides]], [[Brominated flame retardant|brominated flame retardants]], [[alcohol (drug)Ethanol|alcohol]], [[smoking]], and [[Illicit drug|illicit drugs]].<ref name="Salari_2022" /> Among these factors, vaccines have attracted much attention, as parents may first become aware of autistic symptoms in their child around the time of a routine vaccination, and parental concern about vaccines has led to a decreasing uptake of [[childhood immunizations]] and an increasing likelihood of [[Measles#Public health|measles outbreaks]].<ref>{{cite journal | vauthors = Wilder-Smith AB, Qureshi K | title = Resurgence of Measles in Europe: A Systematic Review on Parental Attitudes and Beliefs of Measles Vaccine | journal = Journal of Epidemiology and Global Health | volume = 10 | issue = 1 | pages = 46–58 | date = March 2020 | pmid = 32175710 | pmc = 7310814 | doi = 10.2991/jegh.k.191117.001 }}</ref><ref>{{cite journal | vauthors = Gidengil C, Chen C, Parker AM, Nowak S, Matthews L | title = Beliefs around childhood vaccines in the United States: A systematic review | journal = Vaccine | volume = 37 | issue = 45 | pages = 6793–6802 | date = October 2019 | pmid = 31562000 | pmc = 6949013 | doi = 10.1016/j.vaccine.2019.08.068 }}</ref> However, there is overwhelming [[scientific evidence]] showing that there is no causal association between the [[MMR vaccine controversy|measles-mumps-rubella (MMR) vaccine and autism]]. Although there is no definitive evidence that the vaccine preservative [[Thiomersal controversy|thimerosal]] causes autism, studies have indicated a possible link between thimerosal and autism in individuals with a hereditary predisposition for autoimmune disorders.<ref name="Di_Pietrantonj_2021">{{cite journal | vauthors = Di Pietrantonj C, Rivetti A, Marchione P, Debalini MG, Demicheli V | title = Vaccines for measles, mumps, rubella, and varicella in children | journal = The Cochrane Database of Systematic Reviews | volume = 2021 | issue = 11 | pages = CD004407 | date = November 2021 | pmid = 34806766 | pmc = 8607336 | doi = 10.1002/14651858.CD004407.pub5 | collaboration = Cochrane Acute Respiratory Infections Group }}</ref><ref>{{cite journal | vauthors = Kern JK, Geier DA, Mehta JA, Homme KG, Geier MR | title = Mercury as a hapten: A review of the role of toxicant-induced brain autoantibodies in autism and possible treatment considerations | journal = Journal of Trace Elements in Medicine and Biology | volume = 62 | pages = 126504 | date = December 2020 | pmid = 32534375 | doi = 10.1016/j.jtemb.2020.126504 | s2cid = 219468115 }}</ref> In 2007, the [[Centers for Disease Control and Prevention|Center for Disease Control]] stated there was no support for a link between thimerosal and autism, citing evidence from several studies, as well as a continued increase in autism cases following the removal of thimerosal from childhood vaccines.<ref>{{cite web |title=Timeline: Thimerosal in Vaccines (1999-2010) |url=https://www.cdc.gov/vaccinesafety/concerns/thimerosal/timeline.html |access-date=2024-04-24 |website=CDC}}</ref>
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