I'm nit sure if this makes sense but I'll try my best to explain. Your mind and your body are connected in one way or another and what you think controls what you do. Having a negative attitude can cause you to make negative choices when it comes to health, as in not caring so much so you eat junk food, don't watch your weight, your immune system gets low and etc. Having a more positive attitude has said to cause stronger immune systems and help increase the idea of exercise and healthy living styles. You don't fall into unhealthy habits so easily because you are aware and care for yourself a lot better and you're a lot more careful. That's the best way I can explain it, not sure if it makes sense but I tried. Good luck!
happy=good health(if you are eating and exercising right) depressed= bad health(normally not eating or exercising right)
I think one of the best examples of the mind body connection is the placebo effect. Although this is not an example of "having a positive attitude will improve your health", it is an example where a quantifiable biological effect occurs when people have an expectation about their heath or the treatment they are recieving. Many studies have shown that quantifiable biological changes can be measured when people think they are receiving real medicine, even when they are receiving a "sugar pill". A good place to start looking at this would be wikipedia, as there are many references for primary literature articles there: https://en.wikipedia.org/wiki/Placebo Another thing to check out would be how cortisol levels are related to stress and anxiety. Cortisol is a stress hormone that has a number of detrimental effects (such as decreasing the ability of the immune system to fight disease), and it its production is increased under stress.