Addiction is a lifelong disease involving both biological and environmental components in its foundation. The traditional medical model of disease requires that only that an abnormal condition be present which causes discomfort, dysfunction, or distress to the individual afflicted. The contemporary medical model attributes addiction, in part, to changes in the brain's mesolimbic pathway. The medical model also takes into consideration that such disease may be the result of other biological, psychological, or sociological entities despite an incomplete understanding of the mechanisms of these entities. Within the disease model of addiction, a genetic predisposition is believed to be present. An environmental event is also felt likely to be required.