I just want to add that a reason stress causes health issues is that your body shifts it's attention and energy away from systems in the body that heal, provide maintenance (as well as your immune system as mentioned above) so that it can put that energy toward things that help with fight for flight. Moreover, some of those fight or flight modes, like high blood pressure, can be harmful on their own in the long run. Short bursts of stress aren't unhealthy. It is the chronic, long term stress that wears on your body.