BSC 310 Lecture Notes - Lecture 32: Pneumocystis Jirovecii, Microorganism, Commensalism
Infection and Disease
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
Figure 1
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
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