Autism is a developmental disorder characterized by difficulties with social interaction and communication, and by restricted and repetitive behaviour. The causes of autism are unknown, though it's likely that there is a strong genetic component. Current theory suggests a complex disorder which has core aspects that have distinct causes and often co-occur.
The etiology of autism is complex, and in most cases the underlying pathologic mechanisms are unknown. Autism is a hetereogeneous disorder, diagnosed subjectively on the basis of a large number of criteria. Recent research has investigated genetics, in utero insults and brain function as well as neurochemical and immunological factors.
Source : J. Psychiatry Neurosci. 1999 Mar; 24(2):
Autism is a disorder in which those affected experience impaired social interactions, communication, and behaviors. A new approach to studying autism is to focus on individuals’ perception of self rather than understanding the individual's social interactions. A common thought is that understanding of the differences between the self and others are impaired. However, the exact biological mechanism of self-understanding in autistic children is currently unknown."
Source :Wikipedia
The number of people affected by autism worldwide is also unknown - some countries have no reliable records of its incidence. In countries where it has been possible to measure, the rates tend to vary from around 0.5% to 3%.
Note: In some countries, the degree of prevalence currently appears to be increasing, for which there is as yet no agreed explanation.