Human can harbor many micro organisms on and in side body with out harmful effects. Our skin secretes mucous which keep the pH of our skin slightly acidic. This does not allow many micro organisms to grow on the skin. Inside our body there are many beneficial micro organisms called normal flora of the body. They live in our body as symbiosis. When pathogen attack in side our body, our immune system fight against them in order to kill them by producing antibody and interferon. During this process white cells also die. When there is deficiency of WBC in comparison with pathogen, the infection spread.
