Can optimism affect your health?

I found a great quotation!



Optimism is the faith that leads to achievement. Nothing can be done without hope or confidence. - Helen Keller



And in Learned Optimism by Bernard Seligman, he mentions that optimism leads to better health, which leads to longer life. It seems that the immune system is strengthened by positive thoughts and weakened by negative thoughts. This agrees with another book, You can't Afford the Luxury of a Negative Thought by John Roger.