A delicate relationship exists between pathogenic microorganisms and body defenses. When the defenses resist the pathogens, the body remains healthy. But when the pathogens overcome the defenses, the result is disease. Once disease has been established, the infected individual may suffer temporary or permanent damage or may experience death. The outcome depends upon many factors attending the episode of disease.
The scientific study of disease is called pathology, from the Greek “pathos” meaning suffering. Pathology is concerned with the cause of disease, called the etiology (the agent of disease is the etiologic agent). It also deals with pathogenesis, the manner in which a disease develops. Pathology is also concerned with the structural and functional changes brought about by the disease in tissues.
The terms infection and disease do not have identical meaning. Infection refers to an invasion of body tissues by microorganisms; disease is a change from the state of good health resulting from a microbial population living in the tissues (Figure 1 ). Infection may occur without disease. For example, the flora of microorganisms always present on the body's skin is a type of infection but not disease