Autism prevalence increased rapidly in the late 1980s. The epidemic increased simultaneously in states across the United States, indicating that U.S. children were exposed to toxins in a consistent manner across the entire country. Due to the high adherence amongst the states to the CDC-recommended vaccination schedule, vaccines typically introduce a new exposure to children simultaneously throughout the country.
The phased introduction of the Hib vaccine in 1985-1990 and the universal introduction of the HepB vaccine in 1991 correspond closely with the epidemic increase in autism prevalence. A history of the issue with thimerosal-containing vaccines can be found here.
The graph below shows a correlation between introduction of these mercury-containing vaccines (thimerosal is the name for the ethylmercury preservative in vaccines) and the rise in autism prevalence.