Pathology is the study of the causes and effects of disease or injury. The word pathology also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in the context of modern medical treatment, the term is often used in a narrower fashion to refer to processes and tests that fall within the contemporary medical field of "general pathology", an area which includes a number of distinct but inter-related medical specialties that diagnose disease, mostly through analysis of tissue and human cell samples. Idiomatically, "a pathology" may also refer to the predicted or actual progression of particular diseases, and the affix pathy is sometimes used to indicate a state of disease in cases of both physical ailment and psychological conditions. A physician practicing pathology is called a pathologist.

जिन कारणों से शरीर के विभिन्न अंगों की साम्यावस्था, या स्वास्थ्यावस्था, नष्ट होकर उनमें विकृतियाँ उत्पन्न होती हैं, उनको हेतुकीकारक और उनके शास्त्र को हेतुविज्ञान (Etiology) कहते हैं। ये कारण अनेक हैं। इन्हें निम्नलिखित भागों में विभक्त किया गया है :

• 1. वंशानुगत, जन्मजात या शरीर रचना संबंधी,
• 2. आवश्यक द्रव्यों का अभाव (Deficiency),
• 3. संक्रामक (infectious) उपसर्ग,
• 4. अभिघात (Trauma),
• 5. भौतिक (physical) तथा
• 6. रासायनिक (chemical)।