Etiology: This is the study of origin of diseases. In other words we can say etiology is the study of causation and origination. Etiology most commonly used in philosophical theories or in medical, where it is used to understand that why things occur, or even there are reasons behind the way that things act. Etiology is used in physics, philosophy, government, psychology, medicine, biology or theology. 

Etiology sometimes plays a part of a chain of causation. An etiological agent or a scientist of disease may need an independent co-factor or be subject to a promoter to increases expression to cause disease. An example in the chain of causation, that was recognized late, is the peptic ulcer disease which can be induced by stress and requires the presence of acid secretion in the stomach or has primary etiology in Helicobacter pylori infection.