Understanding illness

Separate cause, mechanism, manifestations and outcome.

To understand an illness, ask four separate questions: what started it, how does it develop, what does it cause, and what may happen next?

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Aetiology

The cause or causes of a disease.

For example

A microorganism may be the cause of an infection.

Read all notes for this section

Aetiology: The cause or causes of a disease. A microorganism may be the cause of an infection.
Pathogenesis: The sequence of changes through which a disease develops. A blocked vessel can reduce oxygen delivery and injure tissue.
Sign / symptom: A sign is observed or measured. A symptom is felt and reported by the person. A measured fever is a sign; feeling nauseated is a symptom.
Prognosis / complication: Prognosis is the expected course. A complication is an additional problem arising during the condition. A complication may change the prognosis, but neither term means the original cause.

Ask four separate questions

To understand an illness, ask: what caused it, how did it develop, what changes can be noticed, and what might happen next? The technical terms are aetiology (cause), pathogenesis (development) and prognosis (likely course). Learn the questions first, then attach the names.

Separate what is felt from what is measured

A symptom is something a person experiences and reports, such as nausea or pain. A sign is something observed or measured, such as a temperature reading. Both are useful information. Neither alone necessarily identifies the cause. A risk factor increases the chance of illness; it does not prove why one person became ill.

Do not confuse timing with severity

Acute usually describes a rapid onset or short course; chronic describes persistence. Neither word tells you severity by itself. Congenital means present at birth, which is not always the same as inherited. Inflammation is a response to injury and may happen without infection.

Explore the illustration

Look at the whole view first. Then select one label and connect it to the idea you just read.

From obstruction to tissue injury

One example of pathogenesis: obstruction reduces blood flow, which can deprive downstream tissue of oxygen.

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01 / 4 Flowing blood

An open vessel delivers oxygen-bearing red cells to the downstream capillary bed. Oxygen passes from the blood towards the supplied tissue.

Schematic sequence: a wall-attached thrombus restricts the downstream supply. Vessel and cells are enlarged for teaching; colours indicate oxygen deprivation, not measured tissue oxygen levels. Reduced motion is enabled. Use the arrows to study one step at a time, or choose Play.

Flowing blood

Red cells transport oxygen along the vessel towards tissues.

Go deeper

In-depth reading

Follow this example step by step, then try explaining it yourself. The extra reading is optional: you can return to it after exploring the illustration.

Worked example: a blocked blood supply

A clot forms inside a blood vessel: this is thrombosis. It may remain at that site. If material breaks away and travels, the travelling material is an embolus; an obstruction caused by it is an embolism. Not every embolus is a clot.
If the blockage reduces blood reaching tissue, the tissue receives less oxygen and nutrients. This inadequate blood supply is ischaemia. Remember the transport lesson: a cell depends on delivery, not simply on the presence of oxygen in the lungs.
Severe or prolonged ischaemia can cause tissue death, called infarction. These words describe different events, not four names for the same thing. A clot does not always lead to every later step.

Explain it yourself

A person reports nausea and has a measured raised temperature. Which is the symptom?

Show the explanation

Nausea is the symptom because it is experienced and reported. The measured temperature is a sign. These findings need context before a cause can be identified.

More detail — when you are ready

These notes expand the mechanisms and vocabulary. Read one topic at a time and find the structures in the illustration.

A framework for explaining disease

Aetiology identifies causes; pathogenesis explains the chain of events linking a cause to altered structure and function. Prognosis estimates the likely course, while a complication is an additional problem arising during disease.

A symptom is a personal experience such as nausea. A sign can be observed or measured, such as a recorded temperature.

A syndrome is a recognisable combination of findings, which need not have a single cause. “Idiopathic” indicates that a cause has not been established; it does not mean that the symptoms are imaginary.

Separate cause, timing and mechanism

Genetic abnormalities may be inherited or acquired. Congenital means present at birth and does not prove inheritance.

Acquired means developing after birth. Communicable conditions can pass between individuals; iatrogenic conditions result from healthcare intervention.

Acute and chronic describe time course rather than severity: chronic disease can be severe, and an acute condition need not be fatal. Infection, trauma, toxins and immune dysfunction are examples of causes or initiating influences.

A risk factor changes probability but does not prove why a particular person became ill.

Worked mechanism: clot, travel and tissue injury

Thrombosis is clot formation within a vessel. If material travels and obstructs another vessel, the event is an embolism; not every embolus is a blood clot.

Reduced blood supply produces ischaemia, and sufficiently severe or prolonged ischaemia may cause infarction, meaning tissue death. These terms describe successive possibilities, not interchangeable names.

Other disease processes include inflammation without infection, degeneration with loss of tissue function, metabolic disruption, abnormal cell growth and autoimmunity. A tumour is not automatically malignant, and a suffix alone cannot establish a diagnosis.