When living organisms such as germs or viruses invade the human body, the tissues of the body undergo changes which help them to resist the poisons of the invader. By this reaction the tissues become immune to the poisons. Much depends on the virulence of the infections, the total number of germs invading, the place where they enter the body, the tissues or structures where they settle and grow. If you are susceptible to the infection, it will attack you; it may even overcome you. If you are resistant, the tissues of your body may develop antibodies which will overcome the germs or viruses or their poisons.

Certain environmental conditions may increase or lower your resistance to infection. Chilling of the body, excessive fatigue, absence of some essential nutritional substance, as proteins, or mineral salts or vitamins, or the presence of another disease at the same time may modify the resistance of the body to an invader.