I’m pretty sure with stress (anyone correct me if I’m wrong) is that it’s a precipitating risk factor. It contributes to the occurrence of a mental disorder, by influencing us to believe that we do not have the resources to cope to everyday stressors. It can then contribute to the occurrence of mental phobias. For example, if we see a spider, it can initiate the flight-fight-freeze response (a stress response) and thus we have a vivid memory of that stimuli and henceforth implement avoidance strategies.
Moreover, stress entails psychological and physiological arousal. Our body is therefore not in homeostasis, and in the long run, can contribute to heart diseases and high blood pressure (release of glucose).
Oh yeah, and also remember sometimes stress can be good! When we experience Eustress, that can contribute to a better mental health as we are able to function at potentially our optimum peak via adrenaline, increased glucose etc.
Mod edit: merged double post. In future, if you have extra information to add please edit your original post rather than posting twice in a row