Most human diseases in the past have originated from infection by a bacteria or viruses. Such diseases are no longer a serious threat to humanity thanks to the development of vaccines, antimicrobial drugs and improved hygiene. Despite these advances, humankind today is burdened with a new dimension of chronic diseases of the immune system, such as autoimmunity, allergies, atherosclerosis, Alzheimer’s disease, diabetes, and cancer. Intriguingly, the improved medical care and hygiene appear to increase, rather than decrease, the incidence of these medical problems.