For decades, science has been unable to uncover the underlying cause of autism. Many theories have been offered yet none has been verified. None of the environmental factors that have been suspected of having a causative role in autism, including certain foods, infectious diseases, pollutants, pesticides, vaccines and illicit drugs, has been proven to cause autism or to increase the risk of developing it. Recently, progress has been made. Although we still do not have a complete understanding of what causes autism, nonetheless there have been several important breakthroughs over the past five years which have begun to unravel the mystery of autism. Scientists have recently discovered a number of rare gene mutations which have been shown to lead to autism. These mutations only explain a small minority of cases of autism, but their discovery has lead researchers to believe that genetics likely plays a large role in the other cases of autism as well. It is most plausible that an individual who has a genetic predisposition toward autism can have his autistic tendency triggered by certain environmental stressors that occur during conception, pregnancy or infancy, even though those environmental factors may not harm someone who does not have a predilection towards autism. In addition, some research points to the immune system playing a role in this process, and the interplay between genetics, environmental stressors, and a maladaptive immune response to these stressors may be central in the development of autism. There is much research currently ongoing to further elucidate the relative roles that these factors may play in the development of autism, and how the interaction between several of these factors may lead to the structural changes in the brain that is found in autistic individuals.