People get infectious disease when microorganisms overpower our host defenses, i.e., when the balance between the organism and the host shifts in favor of the organism. The organism or its products are then present in sufficient amount to induce various symptoms, such as fever and inflammation, which we interpret as those of an infectious disease. From the organism's perspective, the number of organisms and the virulence of these organisms are two important determinants. Clearly, the greater the number or organism and its virulence, greater is the likelihood of infection. From the host's perspective, the two main arms of our host defenses are innate immunity and acquired immunity. A reduction in the functioning of any component of our host defenses shifts the balance in favor of the organism and increases the chance that an infectious disease will occur. Some important causes of a reduction in our host defenses include genetic immunodeficiencies such as agammaglobulinemia; acquired immunodeficiencies such as AIDS; diabetes; and drug induced immunosuppressant. In many instances, a person acquires an organism but no infectious disease occur s because the host defenses were successful.
