Skip to main content

Diagnosis: understanding illness

Published: 11 July 2019

The first step towards treating an ill person is finding out what is causing their symptoms. The process medical practitioners use to determine what is affecting their patient is called diagnosis.

Being unwell can be an overwhelming and frightening experience, especially if you don't know why you feel ill. The need to understand the causes of illness is deeply ingrained. Knowing what is happening to you, and what is likely to happen next, can give you back some sense of control over your body. 

For much of history, finding an explanation for disease was as much a cultural and spiritual concern as it was a biological one. The health of the spirit was associated with the health of the body. 

Sickness might be the result of something you did or didn't do, or of your environment or the alignment of the stars. It might also be blamed on outsiders and vilified groups such as 'witches' or foreigners.

Medieval zodiac man diagram Wellcome Collection (CC BY 4.0) Image source for Medieval zodiac man diagram
Zodiac man used for medical astrology, 1580

People rarely consulted doctors. For the most part they found their own explanations for being ill and produced their own remedies, often passed down from one generation to the next. When the advice of a healer or physician was sought, patient and doctor would construct a narrative between them for the cause and nature of the illness.

Until the 1800s, medical and lay people mostly shared the same language for illness and disease. The doctor's diagnosis was largely based on the patient's account of their symptoms and the course of their disease. Physical examinations were limited, if they happened at all.

Bronze bust of Hippocrates Science Museum Group Collection
Bronze statue of Hippocrates, 1860–1920

Diagnosis was often challenging because many diseases present with similar signs and symptoms such as a rash or a fever—a challenge still faced by doctors today. Even if a physician correctly identified a disease, treatment was usually limited to bloodletting and purging with emetics and laxatives. 

But from the earliest times, doctors appreciated the importance of being able to describe the symptoms of a disease and its likely progress in order to gain patients' trust and valued custom. In his Book of Prognostics, the ancient Greek physician Hippocrates declared:

It appears to me a most excellent thing for the physician to cultivate Prognosis; for by foreseeing and foretelling, in the presence of the sick, the present, the past, and the future, and explaining the omissions which patients have been guilty of, he will be the more readily believed to be acquainted with the circumstances of the sick; so that men will have confidence to entrust themselves to such a physician.

Specific symptoms for specific diseases

From the end of the 1400s, a revival of interest in anatomy saw medicine focus on the body as the main field of enquiry about disease. And in the 1600s, the rise of natural philosophy and the systematic study of nature also filtered into medicine.

The 1600s English physician Thomas Sydenham (1624-1689) was interested in a more empirical approach to medicine. He thought medicine should focus on the physical signs of disease rather than narrative descriptions because:

Nature, in the production of disease, is uniform and consistent, so much so, that for the same disease in different persons the symptoms are for the most part the same

Herman Boerhaave by J Chapman

Sydenham’s approach encouraged his successors to classify diseases by their symptoms so that specific therapies might be applied to specific diseases.

Hermann Boerhaave (1668–1738) was an admirer of Sydenham, and Boerhaave’s clinical approach to medicine and teaching at the bedside also influenced 1800s hospital medicine.

For physicians in the 1800s, particularly those following the French school of medicine, the main aim of their profession was not to cure disease, but to give an accurate medical diagnosis and a satisfying prognosis of a patient's chances.

Only several decades later did the focus of Western medicine shift to curing disease.

Differential diagnosis

Hospital medicine in the 1800s provided an opportunity to observe and compare the prognoses of large numbers of patients with the same disease, resulting in a more reliable system of identifying and classifying diseases. 

Differential diagnosis consisted of:

  • Identifying external symptoms by clinical observation and physical examination

  • Carrying out a pathological examination after death to identify internal lesions that correlated with external symptoms

  • Comparing many cases of the same disease to establish a consistent pattern of symptoms and lesions that defined the disease

These steps remain the basis of disease identification today, although a fourth step, laboratory analysis, would be included.

Doctors use differential diagnosis as a technique to distinguish between diseases with very similar symptoms. 

Pierre Louis (1787–1872) was one of the first doctors to use differential diagnosis to distinguish between very similar diseases. He used it to characterise typhoid as distinct from other fever-causing diseases.

Medical practitioners have used various methods and tools for diagnosis. These include their senses, the patient’s account of his or her symptoms, and medical instruments and tests.

Diagnosing mental illness

Psychological or behavioural disorders can be difficult to diagnose. Their symptoms are sometimes less visible, or more ambiguous, than symptoms of physical disease.

The stigma often associated with mental illness creates new challenges for the person diagnosed with it. Diagnosis can also affect your position in society and personal rights. 

And then there is the risk of pathologising behaviour considered undesirable, particularly among marginalised groups. If naming a disease can provide a sense of power over it, medicalising personality traits or an individual's sexuality, for example, can also remove power.

In 1851, an American doctor argued that enslaved Africans were afflicted with a compulsion to escape called ‘drapetomania'. This medicalisation of African people's responses to enslavement was used by some supporters of slavery. They argued that captive Africans should never be treated too well, for fear that they would catch this 'disease'.

Another problem in psychological diagnosis is establishing what counts as a disease. A 1972 study revealed that psychiatrists in New York diagnosed schizophrenia twice as often as psychiatrists in London. Patients from both cities acted in ways that might have been symptoms of either schizophrenia or mood disorder, which have very different treatments and outcomes.

American psychiatrists subsequently narrowed their definition of schizophrenia. They also reorganised the classification of mental illnesses to more reliably match a specific disorder to a specific treatment. This had tremendous consequences worldwide for the practice of psychiatry, thanks to a radical revision of the American Psychiatric Association's Diagnostic and Statistical Manual (DSM) in 1980.

This, and subsequent revisions, show how social attitudes have influenced what is and is not categorised as a mental illness or condition. Some historic diagnoses, such as hysteria—a nervous illness associated exclusively with women—have been excluded from newer versions of the DSM as unworkable. Homosexuality as a diagnosis was removed only after considerable political activism challenged the idea that sexuality was a medical issue.

Whether you are diagnosed with a mental health condition can have far-reaching social as well as medical consequences. A diagnosis of neurosis or shell shock for soldiers during the First World War could mean the difference between receiving medical treatment or a court martial for cowardice or desertion of duty. And being defined as legally 'insane' can affect the outcome of a trial if the defendant is declared not guilty by reason of insanity.

Finally, in the United States and many other countries, health insurers and policy-makers use the diagnostic categories in the DSM to determine what sorts of treatment for mental illness are eligible for reimbursement.

The consequences of diagnosis

Medical intervention doesn’t stop with the diagnosis of a disease or health condition. Medical practitioners must then help a patient decide whether treatment is necessary or desirable, depending on factors such as the availability (and cost) of treatments available, the likely prognosis, risk of side-effects, and the individual's general health and personal situation. 

An early diagnosis might prevent the onset of a more serious condition if a person changes their lifestyle or starts taking preventive medicine. For example, a change of diet might prevent high blood sugar levels from developing into diabetes. But an awareness of risk factors (such as a genetic predisposition to breast cancer) can also be a burden for patients and their families. 

Diagnosis is intimately related to the patient-practitioner relationship. Before the advent of hospital medicine, practitioner and patient were likely to work together to diagnose the patient’s condition. Since the development of hospital medicine and a more scientific approach to diagnosis, doctors have had greater control over the process and course of treatment.

Listen to your patient; he is telling you the diagnosis.

William Osler (1849–1919), Canadian physician

But with access to medical information freely available online, some patients are demanding more involvement in their diagnosis and treatment. In response, medical training has had to address the communication skills of trainee doctors, as well as the ability to absorb vast amounts of medical knowledge. 

Find out more