Microbiology or immunology is the branch of medical science which deals with host-parasite relationship. Secretions of the respiratory tract, GIT, tears, and saliva contain hydrolytic or proteolytic enzymes which usually digest the bacteria that are accidentally taken or inhaled. The gastric juice contains hydrochloric acid, which is a powerful bactericidal agent. So usually bacteria cannot cause infection of the GIT unless the acid level is lowered; it is neutralized by the presence of food and other agents. Human body contains a variety of phagocytic cells such as neutrophils, eosinophils, macrophages, and these are present in all organs of the body. Any microorganism which if enters by accident, these organisms are engulfed by one of these cells and inside the cells the bacteria are digested with the help of digestive enzymes secreted by the phagosomes. If the bacteria succeed in breaking through these defense mechanisms even then the host tissue may not get infected, because of the specific defense mechanism and it is called immunity.