The body triggers a set of neurons in response to stress, which sends signals throughout the body to initiate movement and promote alertness. Repeated exposure to stress can lead to the release of cortisol from the adrenal glands.
Cortisol is both an anti-inflammatory and inflammation system, and too much of it over an extended period of time can cause chronic inflammation and fatigue. While there is a medical phenomenon called adrenal insufficiency syndrome, which is the incapability of the adrenals to product adrenal hormones, most serious MDs don't believe in adrenal burnout.
Overworking and then stopping abruptly can lead to sickness, as seen in post-finals phenomenon in college and caretakers. Breath hold work and adrenaline release can also affect the body.
The Wim Hof Method, a breathing technique that simulates stress response, might have a positive impact on the immune system, as shown by an experiment that induced both mindfulness meditation and the Wim Hof Method to different groups and then injected them with E. Coli. The group who did Wim Hof showed lesser symptoms.