Diathesis-stress refers to the idea that genetic factors may not necessarily be the singular cause a of particular disease. Rather, they may only predispose the individual to the condition. Other factors (such as stress) are required to "trigger" the genetic factor into action.
For example in schizophrenia, where research indicates a relatively strong genetic component, when one identical twin develops the illness the other sibling only has about a 50% chance of developing schizophrenia. This holds true even when the twins are separated at birth and raised in different environments. In other words, some other causal factors must be involved to activate the genetic tendency. Such research has been done in many illnesses and supports the concept of diathesis-stress as an explanation of how a combination of factors produces illness.
Edgar Cayce clearly spoke of the interaction of "heredity and environment" in production of illness. He provided numerous examples of how "pre-natal" and "innate" predispositions are involved in disease. He also described how environment and psychological (psychosomatic) factors can increase our vulnerability to genetic pre-dispositions.
For example, spinal injury can cause the nervous system to become out of coordination, stressing the body's organs and allowing a genetic weakness to become manifest. Or, excessive worry or mental stress might also weaken the body and make it vulnerable to genetic influences.
Of course, in certain instances, the genetic factor is so strong as to almost ensure that a particular problem will manifest. Edgar Cayce usually described such cases as "karmic." Then, the importance of viewing the condition as a lesson to be learned and an opportunity for growth was emphasized.
As a concept, diathesis-stress is important because it helps to explain how the same causal factor (such as a physical or mental stressor) can lead to a variety of illnesses (see nonspecificity). For example, the same physical insult (e.g., spinal injury) might be linked to a variety of disorders, depending upon the genetic predisposition of an individual.