Autism rates in developing countries have risen sharply over the past 20 years. Of children born in 1992, according to the U.S. CDC, approximately one in 150 may be diagnosed with autism spectrum disorder (ASD). Of children born in 2004, about 1 in 68 children will receive a diagnosis of ASD. It is difficult to compare autism rates from the 1990s and later with rates from the 1940s to the 1980s: in previous years, autism was mainly associated with the most affected people and the level of autism was estimated to be only 1 in 10,000 people. Since the 1990s, our understanding of the scope of autism has grown exponentially, and now people who may not previously have been thought to have autism can be classified as one of the various ASDs.
Whether high levels of autism today are due to increased diagnostics and reporting, changes in definitions of autism, or actual increase in the development of ASD is unknown. No matter, researchers and parents who are equally concerned consider the causes of autism, and the problem is widely studied. The role of vaccines has been questioned, as well as other potential risk factors for ASD, such as genetic predisposition, parental age, and other environmental factors. Vaccines have probably gained more scrutiny for any other cause of ASD, and most scientists, physicians, and public health researchers have come to the conclusion that there is no link between vaccines and autism. Some, however, still question whether vaccines contribute to the development of ASD, so public health and medical facilities continue to address these concerns.