The immune system, which is the set of cells and their activity against antigens, or infectious agents that comprises of the body's defense system against diseases. The immune system does a great job of keeping people healthy and preventing infections. Cold and Flu are the most commonly occurring illnesses in the entire world, with more than 1 billion colds per year reported in the United States alone. The common cold is a self-limiting illness caused by any 1 of more than 200 viruses. If you have a weak immune system, you are at the greatest risk to get cold and flu.