Pathology is concerned with the study of the nature of disease, including its causes, development, and consequences with emphasis on the structural changes of diseases. Specifically, pathology describes the origin of disease, its etiologies, and how it progresses and manifests clinically in individuals in order to determine its treatment. Pathology plays a vital role across all facets of medicine throughout life, and currently it extends to the examination of molecules within organs, tissues, or body fluids.

The precise definition of disease is as complex as an exact definition of life. It may be relatively easier to define disease at a cellular and molecular level than at the level of an individual. Throughout the history of medicine, two main concepts of disease have predominated: ontological and physiological [1].

The ontological concept views a disease as an entity that is independent and self-sufficient and runs a regular course with a natural history of its own. The physiological concept, on the other hand, defines disease as a deviation from normal physiology or biochemistry; the disease is a statistically defined deviation of one or more functions from those of healthy people under circumstances as close as possible to those of a person of the same sex and age of the patient. Most diseases begin with cell injury, which occurs if the cell is unable to maintain homeostasis.

Physiology is the study of normal, healthy bodily function. It is concerned with the science of the mechanical, physical, bioelectrical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. It is a broad science which aims to understand the mechanisms of living, from the molecular basis of cell function to the integrated behavior of the whole body.

Pathophysiology is a convergence of pathology and physiology. It deals with the disruption of normal mechanical, physical, and biochemical functions, either caused by a disease or resulting from a disease or abnormal syndrome or condition that may not qualify to be called a disease and now includes the molecular mechanisms of disease. In the year 1839, Theodor Schwann discovered that all living organisms are made up of discrete cells [2]. In 1858, Rudolph Virchow observed that a disease could not be understood unless it were realized that the ultimate abnormality must lie in the cell. He correlated disease with cellular abnormalities as revealed by chemical stains, thereby founding the field of cellular pathology. He defined pathology as physiology with obstacles [2].

Since the time of Virchow, gross pathology and histopathology have been a foundation of the diagnostic process and the classification of disease. Traditionally, the four aspects of a disease process that form the core of pathology are etiology, pathogenesis, morphological changes, and clinical significance [3]. The altered cellular and tissue biology and all forms of loss of function of tissues and organs are ultimately the result of cell injury and cell death. Therefore, knowledge of the structural and functional reactions of cells and tissues to injurious agents, including genetic defects, is the key to understanding the disease process. Currently, diseases are defined and interpreted in molecular terms and not just as general descriptions of altered structure. Accordingly, pathology is evolving into a bridging discipline that involves both basic science and clinical practice and is devoted to the study of the structural and functional changes in cells, tissues, and organs that underlie disease [3]. The use of molecular, genetic, microbiological, immunological, and morphological techniques is helping us to understand both ontological and physiological causes of disease.