Infectious diseases are known to be caused by the invasion and growth in the human body microorganisms; Infection may result from direct contact with patients or from indirect contact. But the human organism is known to have a specific capacity of resistance against infection, which is called immunity, it being natural and artificial. Under various conditions it may be entirely lacking, it may be relative, rarely it may be absolute. A previous attack of many infectious diseases produces a more or less permanent protection against subsequent infection. In the course of their growth in the body, many pathogenic microorganisms produce virulent poisons or toxins, they causing the characteristic symptoms of a particular disease. To meet the infection the cells of the body produce a chemical antidote,