ABSTRACT
Prior to any discussion of disease in an individual organism or popula tion, a conceptual framework of disease needs to be established by defini tion of the pertinent terminology. Therefore, disease can be defined as any definitive morbid condition or process that has a characteristic set of symp toms or qualities. The various aspects of disease include the cause, or etiol ogy, the developmental process, or pathogenesis, the biochemical and morphological alterations of the cells and tissues, and the functional signifi cance or clinical consequence of these alterations. The etiology of disease may be intrinsic (genetic) or extrinsic (acquired); the latter includes infec tious, environmental, toxic, and nutritional etiologies. Neoplastic disease may have an extrinsic and/or intrinsic component, whereas a disease with an uncertain or unknown etiology is referred to as idiopathic disease. Concern ing etiology, disease may be due to a single etiology, such as a highly viru lent infectious agent, or may be multifactorial. For example, a primary and/or secondary infectious disease may occur in fish exposed to poor water quality or low concentrations of a toxin. Disease can be further classified ac cording to the progression and severity of the condition. Acute disease has a rapid onset and progression, whereas chronic disease has a slow progres sion and long duration. Disease that is neither acute nor chronic may be classified as subacute or subchronic, whereas disease that has an extremely rapid progression can be considered peracute. Clinical disease is apparent and characterized by observations and/or the results of tests, whereas subclinical disease is not apparent or does not result in clinical manifesta tions and is difficult to characterize. Subclinical disease may progress to clinical disease.
