According to research in the field of psychoneuroimmunology, upsetting thoughts and emotions may have a negative influence on health because of the link between the brain and immune system.
Psychoneuroimmunology studies the interactions and communications between the brain, the nervous system, and the immune system. It has found that there are bidirectional connections between the mind/brain and the functioning of the immune system.
Negative mental states like chronic stress, anxiety, depression etc. can dysregulate and suppress normal immune responses through neural and biochemical pathways linking the brain to immune cells and organs.
Some key mechanisms include:

1. Stress hormones like cortisol released during emotional distress can directly inhibit immune cell proliferation and inflammatory responses.
2. Neurotransmitters and neuropeptides released during emotional arousals modulate immune cell activity.
3. The brain’s regulation of immune cells and cytokine production is altered by disturbed emotional states.
4. Areas of the brain like the hypothalamus can directly signal the immune system through neural and endocrine pathways.