Involves the interaction of a multiplicity of factors interacting with each other. These factors are the host, agent, and the environment consists of four stages: (a) the stage of susceptibility, which includes a person’s ability to resist the onset of the disease. It is the time when a clinical disease has not yet appeared, but when other factors become present; (b) the stage of presymptomatic disease, which is characterized by the onset of disease but with no clinical signs or symptoms as yet apparent; (c) the stage of clinical disease characterized by overt and recognizable clinical signs and symptoms that can be analyzed resulting in diagnosis; and (d) the stage of disability characterized by either residual defects or the onset of death. A composite of the four stages of the disease is referred to as the natural history of the disease.