Etiology

The causes of disease are known as etiologic factors. Among the recognized etiologic agents are biologic agents (e.g., bac­teria, viruses), physical forces (e.g., trauma, burns, radiation), chemical agents (e.g., poisons, alcohol), and nutritional ex­cesses or deficits. At the molecular level, it is important to distinguish between sick molecules and molecules that cause disease.¹³ This is true of diseases such as cystic fibro-sis, sickle cell anemia, and familial hypercholesterolemia, in which genetic abnormality of a single amino acid, trans­porter molecule, or receptor protein produces widespread effects on health.

Most disease-causing agents are nonspecific, and many different agents can cause disease of a single organ. For ex­ample, lung disease can result from trauma, infection, ex­posure to physical and chemical agents, or neoplasia. With severe lung involvement, each of these agents has the po­tential to cause respiratory failure. On the other hand, a single agent or traumatic event can lead to disease of a number of organs or systems. For example, severe circula­tory shock can cause multiorgan failure.

Although a disease agent can affect more than a single organ, and a number of disease agents can affect the same organ, most disease states do not have a single cause. In­stead, most diseases are multifactorial in origin. This is par­ticularly true of diseases such as cancer, heart disease, and diabetes. The multiple factors that predispose to a particu­lar disease often are referred to as risk factors.

One way to view the factors that cause disease is to group them into categories according to whether they were present at birth or acquired later in life. Congenital conditions are defects that are present at birth, although they may not be evident until later in life. Congenital malformation may be caused by genetic influences, environmental factors (e.g., viral infections in the mother, maternal drug use, ir­radiation, or intrauterine crowding), or a combination of genetic and environmental factors. Not all genetic disorders are evident at birth. Many genetic disorders, such as famil­ial hypercholesterolemia and polycystic kidney disease, take years to develop. Acquired defects are those that are caused by events that occur after birth. These include injury, ex­posure to infectious agents, inadequate nutrition, lack of oxygen, inappropriate immune responses, and neoplasia. Many diseases are thought to be the result of a genetic pre­disposition and an environmental event or events that serve as a trigger to initiate disease development.