Professor Wayne Drevets introduces the diathesis-stress model, which describes the relationship between biology and stress, particularly in relation to mental illness.
The stress-diathesis model often holds that initially the illness gets instantiated by some vulnerability that’s biological with a stressful event, but that once the illness has become instantiated, then it can take on a more spontaneous and recurrent role or nature. In contrast, there are still some relationships with stress. For example, once somebody develops depression, one of the things I commonly see clinically is that when there are severe losses or stressors they can often lead to a stair-stepping or type of worsening of the illness. So for example, the individuals may have had a start to depression with some stressful event and maybe some additional episodes, but then if they lose a parent or a child or have some kind of a severe stressor, it can contribute to a worsened episode that then leads to a more severe course after that. So we don’t really understand the relationship between stressors and illness course, but we do think that there is some interaction at least in some cases between stressful life events and either precipitation of illness or worsening of illness.