Ongoing stress makes us susceptible to illness and disease because the brain sends defense signals to the endocrine system, which then releases an array of hormones that not only gets us ready for emergency situations but severely depresses our immunity at the same time. Some experts claim that stress is responsible for as much as 90% of all illnesses and diseases, including cancer and heart disease. The way it does this is by triggering chemical reactions and flooding the body with cortisol that, among other things, decreases inflammation, decreases white blood cells and NK cells (special cells that kill cancer), increases tumor development and growth, and increases the rate of infection and tissue damage.