What's the difference between etiology, pathogenesis, pathology, pathophysiology and epidemiology?

Question

Most of my searches either end up explaining any of the above words in terms of the other four, or explaining the concept in simple words in such a way that makes it difficult to see the difference between them.

EDIT 1

A simple google search took me to medicinenet.com -

pathology - "that branch of medicine which treats of the essential nature of disease."

etiology - "The study of causes, as in the causes of a disease."

Pathogenesis: "The development of a disease and the chain of events leading to that disease."

So I decided to try wikipedia as, I felt, all definitions will necessarily fail to actually explain the differences between them as the differences are pretty nuanced.

But wikipedia articles on these terms were either very detailed or very general, in the sense that although I understood what they were saying in any particular article, I still couldn't figure out how the rest of the terms were referring to something different. All seemed to mean - understanding why and how diseases work/spread and how to control them.

The only term I understood distinctly here was epidemiology, which means studying the occurrence of diseases in a given population (thus putting a geographical limitation).

I guess I'm looking more for an explanation which draws a parallel to convey the message. Definitions won't work.

For ex, if someone were to ask me the difference between environment, habitat, ecotone and niche, I would try something along the lines of -

Environment = library

Habitat = different sections (like literature, reference, philosophy etc)

Ecotone = gate of library

Niche = the place a particular student occupies in that library through his interaction with other students etc

Answer 1

To get an understanding of the difference, look at a disease that has been in the news recently, Legionnaire's Disease.

Legionella pneumophilia is a bacteria that is responsible for most cases. It lives in stagnant water under certain conditions, and is inhaled when aerosalized. This is the etiology of the disease, which is basically how it infects the host, or how it gets in.

Once inside the body, it starts replicating and causing damage. Initial symptoms include flu like symptoms, headache, fatigue and muscle pain. As it progresses, they can expand to include dry coughing, chest pain, and eventually leading to possible mental status changes, hallucination, etc. This progression of the disease in the various stages is the pathogenesis of the disease, i.e. days 1-3 expect these symptoms, days 4-7 these other symptoms, and so on.

To confirm the diagnosis, the doctors will take samples of various body fluids and send them to the lab for testing, as well as x-rays, etc. One of the tests is staining of the sputum that can show the Legionella bacteria. This is pathology, specifically the examination of body fluid/tissue for diagnostic purposes. (Where it gets a little confusing, is pathology also refers to the body of knowledge about disease cause and effects).

So in a general sense you are correct, they all refer to how/why diseases spread and how to control them, but one doesn't necessarily depend on the other.

Edited to add: Pathophysiology is kind of an intersection, to where the pathology of a condition and the physiology are taken as a whole, and describes the functions and changes associated with a condition from both the observable and the testable views.

Answer 2

Since the OP is asking for definitions, maybe it is OK to be nitpicky.

• Pathogenesis is the process by which harm has occurred.
• Pathology is the study of harm, including the study of pathogenesis.
• Etiology is the investigation of causes. Doctors are most interested in the causes of harm, not in the causes of neutral or good outcomes, so most medical etiology is pathology.
• Pathophysiology is the study of biological processes associated with harm. The etiology of ideopathic conditions is part of pathophysiology. Pathophysiology also includes the study of recovery mechanisms.

Answer 3

I don´t see any issues with JohnP´s answer. I just would approach with different example. Lets take coronary artery disease and cholesterol as an example.

Coronary arteries are probably the most important blood veins in the body since they transport arterial blood from aorta to the heart muscle itself. Coronary artery disease means that heart muscle receives insufficient volume of blood via the coronary arteries and in some instances the blood supply may even be blocked (heart attack or critical ischemia as a milder case).

Why blood flow to heart muscle is reduced. It is due to atherosclerosis. Atherosclerosis is the etiology of coronary artery disease. Atherosclerosis causes the artery walls to thicken and stiffen and when the cross area of the artery is reduced less blood is moved though coronary arteries.

On the other hand the etiology of atherosclerosis is high blood LDL level ("bad" cholesterol). Fat and cholesterol which we receive from food are transported in blood and one transport vehicle is the LDL. LDL is bad since it has ability to get in to the artery wall. In there it causes an inflammation cascade which results to plaques inside the wall. These plaques (=calcifications) are hard are rock and causes the artery wall thicken and stiffen.

The presence of the inflammation and variable sized plaques or calcifications in the artery wall are important aspects in the pathology of the atherosclerosis ad subsequent coronary artery disease. In a healthy heart there are no plaques and the artery walls are intact. Pathology describes what is abnormal in certain diseases.

Cascade resulting to certain pathology or pathological condition is pathogenesis. I consider pathophysiology as a synonym for pathogenesis. I would even introduce a third term: etiopathogenesis. As so, the pathogenesis or pathophysiology or atherosclerosis would be the following: LDL particles are carried in the blood. Once in the coronary arteries, LDL particles penetrate the inner layer of the artery wall. Inside the wall the LDL particles are broken to smaller particles. Marophages invade the wall and they phagocytes (digest) the small LDL particles. These macrophages becomes foam cells which are non-functional fat-laden macrophages just laying in the artery wall. When this process goes on continuously the artery wall starts to thick and cross area of the artery becomes smaller (stenosis).

Epidemiology is a whole another thing. It deals with disease prevalences and incidences in a population level. Following statements from Wikipedia regarding coronary artery disease deals with epidemiology of the disease:

CAD as of 2010 was the leading cause of death globally resulting in over 7 million deaths

Coronary heart disease (CHD) is the leading cause of death for both men and women and accounts for approximately 600,000 deaths in the United States every year. According to present trends in the United States, half of healthy 40-year-old men will develop CAD in the future, and one in three healthy 40-year-old women.