Diseases are symptoms of changes in society

In 1851 William Wilde, physician and father to Oscar Wilde, . He grouped them into categories: "zymotic, epidemic, endemic and contagious diseases", "sporadic diseases" (further divided by the organs they were thought to affect) and "accidents".

That last category was really a catch-all cause of death that included suicide, starvation and drowning.

The individual diseases in each category were a mixture of the familiar (influenza, cancer), the strange (dropsy, ague) and the out-of-place (deaf-dumbness). Deafness is certainly no longer considered a disease.

Wilde's "nosological chart", or classification of diseases, was intended "to induce the officers of public institutions to adopt some general formula for registering the diseases of the poor". To this end he provided English and Irish names for each condition.

Wilde’s list, like the modern International Classification of Diseases, was particularly aimed at improving the records of mortality. This, he felt, would help physicians and the state to better address diseases that were preventable.

Humans like categories, and there is probably nothing so revealing of how our societies change over time than how our categories change with them. My disease, for example, does not appear on Wilde's list. I have hypertension or high blood pressure. It is known as the "silent killer" because it causes few noticeable symptoms and slowly causes wear and tear to your blood vessels. Then one day, bang, you have a stroke.

Or maybe you don't. Maybe, instead, you get run over by a bus and, having donated your body to science, the anatomy student who dissects you discovers the signs of hypertension. But let's transport my cadaver back in time: did I still have hypertension when that disease category didn't exist? Transport Wilde forward in time to study my cadaver and he would probably find evidence of a disease that, according to modern medicine, no longer exists. All this leads to a question that historians have long considered: what exactly is a disease?

You are probably thinking that you know what a disease is: it makes you sick. Once medical science could detect hypertension it became a disease and treatments were developed. But that is not exactly how it works. To paraphrase the historian Charles Rosenberg, for a disease to exist we have to agree that it does. "We" includes not just doctors but patients and the society and culture that shape them.

The technology to detect blood-pressure changes has been in existence since at least the 1730s. Up until the mid-20th century, many doctors did not agree that hypertension was a disease to be treated. Some thought it was normal human variation, others that it was a physiological response that should not be interfered with. To prove that it was a disease, enormous studies of thousands of people were undertaken and a correlation was made between high blood pressure and deaths from stroke and heart disease.

The discovery of hypertension in the 20th century also involved companies that began screening applicants for life insurance and the changed expectations of the public around life, health and disease. Hypertension became a disease not just because doctors could identify it, but because we all agreed that good health meant not dying of stroke in your 40s.

I am not saying that sickness and health are relative: I take my tablets. But we should remember that there are lots of factors influencing when and why a disease emerges.

In Wilde's time "fever" was an extremely common cause of death and disease among the poor of Ireland. Fever for us is a symptom of another disease. Fever for Wilde was a disease caused by insanitary living conditions and poverty. "Fever" stood not just for a list of symptoms, it stood for a social problem: the poor died in higher numbers and at younger ages than the wealthy. Wilde did not see fever as a natural check on population or a consequence of intemperate lifestyles but as a reason to reform society by alleviating poverty and improving sanitation.

Emerging diseases are not just ways to get sick, they are also symptoms of social changes and ideas about social reform.

• Juliana Adelman lectures in history at St Patrick's College, Drumcondra