The body must contend with constant attacks by microscopic organisms. In order to defend against this onslaught, it deploys a wide range of defenses that together are called the immune system. People with diseases that cause immune deficiency fall victim to infectious microorganisms that a healthy person could ward off easily. However, even healthy people get sick from time to time, victims of infections that manage to sneak by the defenses. And some apparently healthy people nonetheless get sick quite often. If you fall into this latter category, you may wish to find treatments that can strengthen your immune system. Unfortunately, this is easier said than done. To explain why it is so difficult to improve resistance to illness, we need to delve a bit deeper into the nature of immunity.