There is a growing literature describing the stress-vulnerability model of illness, a model applicable to many, if not most, psychiatric disorders and to physical illness as well.  Vulnerability comes in a number of forms.  Genetic predisposition to specific conditions may arise as a result of spontaneous mutations, or it may be transmitted intergenerationally in family pedigrees.  Secondary types of vulnerability may involve susceptibility to disease caused by the weakened resistance that accompanies malnutrition, immunocompromised states, and other conditions.