11Introduction to Clinical Applications Objectives In this chapter we will study •various approaches to the study of disease; •the role of the Centers for Disease Control and Prevention; •common causes of disease; •the distinction between signs and symptoms of disease; •terms used to describe the time course of a disease; and •common abbreviations for medical specialists and specialties. Homeostasis and DiseaseThe body’s tendency to maintain internal stability is calledhomeostasis.Examples include the body’s relativelystabletemperature,bloodglucose concentration, hormone levels, acid-base balance, and electrolyte balance. When physiological variables deviate too much from theirset point,the body activatesnegative feedback loopsthat tend to restore stability and maintain health. In some cases, such as the stoppage of bleeding,positive feedback loopsare activated to bring about rapid change. If the attempt to regain homeostasis fails,diseaseresults. There is a strong emphasis in medicine today on promoting wellness through prevention. However, this manual focuses on what happens when prevention fails, homeostasis is disrupted, and disease occurs. The Study of Disease Disease (illness)is any deviation from normal that interferes with correct, life-sustaining bodily function. Literally, the word meansdis-ease,the opposite of ease (comfort and normal function). Disease may have underlying structural foundations, such as a broken bone, and its effects may be observed not just at the level of bodily form and function but also at the level of the mind, as in psychiatric diseases (mental illness). The study of disease is calledpathology,a field that embraces all aspects of disease, from the patient’s complaints to the gross and microscopic appearance of dysfunctional tissues and organs.Pathologistsare physicians and others who specialize in this branch of medicine.Asubdivisionofpathologycalled pathophysiology focusesspecificallyonthe physiological(functional)aspectsoforgan dysfunction,asopposedtotheirstructural abnormalities.Histopathologyisthestudyof diseased organs at the microscopic level. Epidemiologistsare scientists who study the social distribution and spread of diseases, especially to determine their sources and causes and to halt their spread. Since epidemiology is such an important public health concern and epidemiologists play a key role in formulating public health policy, many epidemiologists work at such organizations as the World Health Organization (WHO), U.S. Public Health Service (USPHS), and comparable national health agencies in other countries. One of the premier institutions for epidemiology is the USPHS division called the Centers for Disease Control and Prevention (CDC), headquartered in Atlanta, Georgia. The CDC was originally established in Georgia because of the prevalence of malaria in that region of the United States and the importance of this infectious disease to the U.S. military personnel who trained at bases in the