Epidemiologic patterns suggest that viruses, nutrition, toxic agents or socioeconomic psychosocial factors may contribute to the etiology alone or in combination. Elucidation is confounded by the long interval between exposure and onset of clinical disease, as well as the interaction of multiple genes and/or insults, which appear to interact in a complex manner. Numerous studies have investigated environmental influences but have yielded conflicting results. This may be in part due to the failure to account for genetic susceptibility, begin observation at early ages or in utero, and/or monitor subjects long term and frequently.