Most kids diagnosed with autism do not have serious cases. Diagnostic expansion is the source of the autism increase, and social pressure to conform with limited physical activity. Kids who are truly autistic in the serious sense are very rare. I have students all the time who are diagnosed as autistic but those with serious problems like you read about in the newspaper occasionally almost never reach this level.
The WHO says, "Based on epidemiological studies conducted over the past 50 years, the prevalence of ASD appears to be increasing globally. There are many possible explanations for this apparent increase, including improved awareness, expansion of diagnostic criteria, better diagnostic tools and improved reporting."
A study in the NIH:
"Following an expansion of diagnostic criteria for autism that occurred in the late 1980s and 1990s, autism prevalence studies around the world showed dramatic increases (Fombonne, 2009; Rice, 2013; Rutter, 2005)."
"It is likely that the rise in autism prevalence during the latter decades of the 20th century, based on epidemiologic studies, can be attributed largely to the expansion of diagnostic criteria and the adoption of the concept of autism as a spectrum of impairments (ASD) that occurred during this period (Fombonne, 2009; King and Bearman, 2009; Rice, 2013; Wing and Potter, 2002). It is also possible that other factors, including improvements in screening and services for children with ASD and increases in specific risk factors for ASD (such as increases in the proportion of births to older parents) have also contributed to increases in the prevalence of ASD over time (Durkin et al., 2008; Grether et al., 2009; Rice, 2013; Rice et al., 2013; Schieve et al., 2011)."
"Taylor et al. (1999) conducted a self-controlled case-series study in children with autistic disorders residing in the North East Thames region of the United Kingdom. ... A total of 498 children who were born from 1979 through 1998 and had an autism diagnosis before 16 years of age were included in the analysis. ... The authors concluded that MMR vaccination is not associated with autism.
Farrington et al. (2001) conducted a reanalysis of the study by Taylor et al. (1999). The two risk periods were changed to autism diagnosis within 59 months and any time after vaccination, and compared to a reference period that consisted of time from birth through 191 months of age or August 1998, whichever occurred first. ... The authors concluded that there is no association between MMR or measles-containing vaccines and autism diagnosis any time after vaccination.
Madsen et al. (2002)2 conducted a retrospective cohort study in children born in Denmark from January 1991 through December 1998. ... The authors concluded that MMR vaccination is not associated with an increased risk of autistic disorder or other autistic-spectrum disorders.
Smeeth et al. (2004) conducted a case-control study in children (born between 1973 and 1999) enrolled in the General Practice Research Database (GPRD) from June 1987 through December 2001. The study included 991 cases with a recorded diagnosis of autism and 303 cases with other pervasive developmental disorder diagnosis. ... The authors concluded that MMR vaccination is not associated with an increased risk of autism.
Mrozek-Budzyn et al. (2010) conducted a case-control study in children identified in the general practitioner records in the Malopolska Province of Poland. The study included 96 cases and 192 matched controls. The cases were diagnosed with childhood or atypical autism by a child psychiatrist according to the ICD-10 criteria. ... The authors concluded that administration of MMR or single-antigen measles vaccine is not associated with an increased risk of autism in children.
https://www.ncbi.nlm.nih.gov/books/NBK332896/