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
The balance between health and disease. The condition of the human body depends on interactions between host (the body) and parasite (the infectious microorganism). When the human body wins the battle for supremacy, the result is health and the rejection of disease.
The normal flora. The normal flora is the population of microorganisms found where the body tissues interface with the environment. Much of the normal flora is permanent, but some portions are transient. The transient flora is present for a time and then disappears.
Various types of relationships exist between the normal flora and the body. The general name of a relationship is symbiosis, a term that means living together. In some cases, the symbiosis is further identified as a commensalism, when one organism benefits and the other remains unaffected. A type of symbiosis called mutualism exists when both organisms benefit one another. A symbiosis called parasitism develops when one organism damages the other.
Opportunistic organisms. Certain organisms of the normal flora are opportunistic.Opportunistic organisms are potentially pathogenic organisms that normally do not cause disease. However, in a compromised host, the organisms may see “opportunity” to invade the tissues. An example occurs in individuals who have AIDS. Opportunistic organisms such as Pneumocystis carinii invade the lung tissues and cause a lethal pneumonia.