Medical researchers at the Institute of Heartmath, led by neurochemist Dr Glen Rein (with whom I worked over 30 years ago), published a fascinating report in the Journal of Advancement in Medicine that showed how positive and negative thinking can affect your immune system.
They monitored the amounts of salivary immunoglobulin A (s-IgA) after each person in the study thought about 'care and compassion' or 'anger and frustration'. s-IgA is a part of the immune system and is found in the saliva that can neutralise the bacteria that enter your mouth, from food for example. A high level of it indicates a strong immune system, whereas a lower level indicates a weakened immune system.
The Heartmath scientists discovered that 'care and compassion' produced a stronger immune system than 'anger and frustration'. They found that just five minutes of 'care and compassion' caused the immune system to be elevated for five hours, whilst five minutes of 'anger and frustration' depressed it for five hours.
It's a great demonstration that you can create positive states of health by thinking positively.