disease

disease can be broadly defined as any illness or sickness that impairs or disrupts the normal functioning of the human body. Definitions of disease, however, have varied over time and place, and diagnostic categories vary. We will be concerned here with concepts of disease in the Western medical tradition. Medicine itself can be defined as ‘the science and art concerned with the cure, alleviation, and prevention of disease, and with the restoration and preservation of health’. Thus the definition, treatment, and prevention, of disease is intimately associated with the development of medicine itself, and several specialist areas of medicine are specifically concerned with broad aspects of disease. For example nosology, the classification of diseases, was for many centuries the cornerstone of medicine; therapeutics is the treatment of disease; diagnosis is the art the physician uses to determine from the patients' signs and symptoms what the underlying causative mechanism may be; epidemiology is the study of disease in populations.

The Hippocratic tradition maintained that diseases were physiological, arising from an imbalance between the four humours, the correct balance of which maintained health. Each individual had a unique humoral balance which could be easily disrupted by conditions such as cold, biting winds, poor air, or injudicious eating. It was Paracelsus (c.1492–1541) who provided an alternative to such classical ideas by suggesting that disease was the product of active agents independent of the human patient. The English Hippocrates, Thomas Sydenham (1624–89), also believed that diseases were specific entities, which might be manifest in variable ways in individual patients, but which could be recognized by observant clinicians. Sydenham also advocated that specific remedies could be applied to each such disease, his favourite example being the prescription of Peruvian bark for intermittent fever or ague. It was during the eighteenth century that the classification of diseases became a dominant part of medicine, such taxonomies often being based on the presenting symptoms. Increasing attention to pathology and cellular mechanisms during the nineteenth century provided additional criteria whereby diseases could be described, recognized, and treated.

An increasing array of diagnostic techniques have become available during the twentieth century, and these have made it possible for even more diseases, syndromes, and conditions to be classified, and the International Classification of Disease goes some way to providing international standardization in the categorization of modern disease.

Disease categories

One broad classification divides diseases into two principle categories, defining disease as primarily congenital (present at birth) or acquired subsequently. The acquired category can be further subdivided to include infectious, neoplastic, traumatic, and degenerative diseases, which are not necessarily mutually exclusive. Modern concepts and definitions have obscured many of these classificatory boundaries, and occupational, nutritional and deficiency, autoimmune and allergic, and psychiatric diseases can now be included in contemporary nosologies. As the molecular mechanisms of diseases and their causations are increasingly understood, so classifications are increasingly becoming blurred and overlapping, and many diseases are now recognized as being multi-causal. The following sections will give brief overviews of some broad classes of disease.

Genetics and diseases

Hereditary diseases may be passed down from generation to generation, but are not necessarily genetic disorders unless determined by one or more genes. The experiments on peas by Gregor Mendel (1822–84), on the transfer of characteristics from generation to generation, established the basic principles of heredity. Mendel and his successors determined a number of factors to define genetic inheritance, including its occurrence in known proportions amongst relatives, but not in unrelated individuals, such as in-laws. Applied to human conditions, some diseases were now identified as hereditary, including haemophilia and sickle-cell anaemia — although it had been recognized for centuries that some diseases ‘ran in the family’.

congenital abnormalities are present at birth, though some do not immediately become apparent. They are not necessarily inherited, many in fact being environmentally determined during intrauterine life. It is now known that many common diseases, including heart disease, insulin dependent diabetes, some forms of psychiatric illnesses, and autoimmune diseases, have a genetic component. But these susceptibilities are often triggered by some environmental influence, and represent a complex interaction between nature (genetic makeup) and nurture (external influences). Some forms of cancer are increasingly recognized as having a major genetic component. The genetics of some diseases, such as cystic fibrosis, beta-thalassaemia, and Duchenne's muscular dystrophy, are now well known, and it is possible to screen parents, and unborn fetuses, to detect genetic abnormalities, and to offer termination of an affected fetus. The development of in utero gene therapy offers the hope of treatment, whilst modern medicine can do a great deal to maintain individuals who, in earlier periods, would have died because of their genetic constitution.

Infectious diseases

infectious diseases caused by microorganisms or parasites have been powerful forces in shaping human history. As early humans formed hunter–gatherer societies in about 3000 bc and began to contain and domesticate wild animals, so they became susceptible to the infections carried by the animals with which they now shared their living space. Smallpox, distemper, and measles are amongst the diseases known to have entered human populations at this time. The devastating effects of these diseases gradually became ameliorated as immunity built up in communities — major epidemics (and pandemics) were often caused by the movement of communities where such diseases were endemic into non-immune populations. Urbanization provided fresh opportunities for infectious diseases to flourish and to decimate populations — close living conditions encouraged the transfer of infections; migration into the cities from rural communities, and the dependency of urban populations on the countryside for food, provided fresh avenues for infection. Until almost the end of the nineteenth century, sustaining cities was a constant problem, and the large metropolitan areas of Western civilization were known to be centres of disease, malnutrition and starvation, and ultimately death. Diseases such as smallpox, syphilis, typhoid fever, and whooping cough were all endemic and accounted for a high infant mortality — estimates have suggested for example that the mortality rate for children under 5 was as high as 50% for much of the nineteenth century in the English city of Manchester. By the latter half of that century, the experiments of Robert Koch (1843–1910) in Germany and Louis Pasteur (1822–95) in France, amongst others, increasingly provided evidence that microganisms were the cause of several infectious diseases. Efforts were made to utilize this knowledge in the manufacture of vaccines, preparations of modified or killed bacteria that could be administered to produce a mild form of the disease and to confer immunity. Although the precise mechanisms of how such immunity was created were unknown, a number of therapeutic substances were developed, the biggest breakthrough coming at the end of the nineteenth century with the appearance of serum anti-toxins. Since then, increasing understanding of the underlying cellular mechanisms of immunity and of the biology of microorganisms; the growth and development of the pharmaceutical industry, especially the discovery of antibiotics; and the development of public health measures to prevent and treat infectious diseases, have led to a notable decline in mortality and morbidity from such diseases in the Western world. The same cannot be said for the developing world, and concerns grew at the end of the twentieth century about resistance to antibiotics, the appearance of new infectious killers such as HIV (which causes AIDS), and the re-emergence of drug-resistant forms of diseases such as TB.

Autoimmune diseases

autoimmune diseases occur when elements of the immune system, normally responsible for recognizing and attacking ‘non-self’ cells — such as the microorganisms that cause infectious diseases — fail to distinguish between ‘self’ and ‘non-self’. Such cellular attacks on healthy constituent parts of the body can contribute to a variety of disorders, including myasthenia gravis, some thyroid disorders, and rheumatoid arthritis. There is growing evidence that conditions such as diabetes and multiple sclerosis also have an autoimmune component.

Diseases associated with food: deficiencies, excesses, and intolerances

Deficiency (nutritional) diseases arise from lack of one or more essential nutritional component, such as a vitamin or mineral, in the diet, or because of the body's inability to digest, absorb, or utilize particular nutrients. A nutritional deficiency can also occur if the body's metabolism is abnormal or if essential elements are excessively excreted. Historically, deficiency diseases have arisen in populations forced, by war or famine, to abandon their traditional diets, or by the adoption, perhaps for religious reasons, of a restrictive diet. Expeditions into new territories have always been vulnerable to dietary diseases because of the difficulty of carrying adequate supplies. The most notable example was that of scurvy, and the development of its treatment by eating citrus fruits, which occurred long before the rationale was understood — namely that this corrected a vitamin C deficiency due to lack of fresh fruit and vegetables. Subtle changes to farming or cooking methods can also lead to unexpected deficiencies. One of the best known examples is the use of white (huskless) rice instead of brown (husked) rice. This can lead to beri-beri, particularly prevalent in the Far East, which is characterized by ascending weakness in the legs and accompanying muscle tenderness, and can lead to widespread nerve irritability and congestive heart failure. This is due to a lack of thiamine, a vitamin that is essential for the mechanisms by which energy is released from foodstuffs.

Excess consumption of certain kinds of foods has been shown to be associated with the onset of conditions such as heart disease or diabetes — often exacerbating a genetic predisposition, and thus once more blurring the distinctions between different disease categories. Over consumption of alcohol, cigarette smoking, and taking other damaging drugs can all lead to disease conditions that can be classified as ‘self-inflicted’ (a category that can also include sexually transmitted diseases contracted during unprotected sex).

Food intolerances have been increasingly recognized in the latter part of the twentieth century, as some individuals show anaphylactic responses to particular allergens, such as nuts or dairy products.

Occupational diseases and the effects of pollution

Concern about the workplace as a source of disease has grown, particularly since World War II, as have the specialities of public and occupational health. Safety procedures, including the use of protective clothing, have been proposed to limit workers' exposure to dangerous chemicals or to hazardous practices. Working conditions, such as those for office workers using video display equipment, have received attention, and exposure to noise, bad ventilation, and poorly designed furniture have increasingly been recognized as playing a role in stress. Stress in turn is recognized as contributing to high blood pressure, heart disease, and stroke.

These concerns are not, however, entirely new. Lead poisoning amongst miners was recognized by Hippocrates (c.450 bce), but it was really the impact of the Industrial Revolution that focused the attention of reformers and some physicians on the impact of working conditions on health. A Leeds physician, Charles Thackrah, wrote The effects of arts, trades and professions on health and longevity (1832), which described the occupational hazards attached to numerous trades, including flock dressers, maltsters, coffee grinders, and corn-sillers. The effects of adverse and dangerous conditions on the ordinary working man became an issue for the growing number of trade or labour unions, and reform movements throughout the twentieth century campaigned for safer working conditions and adequate health care and compensation for those injured in the workplace.

Increasingly, however, environmental dangers have been recognized as having wider impact than just at the workplace. There can be pollution from accidental contamination and from large-scale industrial accidents. Disease and disasters can arise from cynical exploitation by manufacturers who ignore concerns for the welfare not only of their own workforce, but also of those living in the vicinity of their production facilities, such as the workers in the asbestos industry or the victims of the Bhopal explosion in India that killed 2000 people. Dispersal of pollutants, by air as after the Chernobyl disaster in Ukraine, or by river systems, can cause disease at vast distances from the original site of contamination.

Psychiatric diseases

Mental illness can refer to disorders in perception, understanding, emotion, and behaviour, and can range from the milder psychological disorders and psychosomatic illness to the severe psychosis. Psychiatric disorders have not always been seen to be the province of the medical profession: theories about demonic possession, for example, have led to religious remedies or persecution. For many centuries doctors had little to do with those classed as ‘insane’. The insane were incarcerated and contained, rather than treated. In the twentieth century increasing acknowledgement of the interplay of social, psychological, and physical factors in the causation of many psychiatric disorders, and the development of specific pharmacological therapies, led to improved care. Here again, the categories of disease classifications have become blurred, as faulty chemical processes in the brain and genetic defects have been shown to account for some manifestations of mental disease.

Degenerative diseases

Ironically, as infectious diseases were increasingly conquered during the twentieth century, degenerative diseases emerged in the West-ern world, primarily affecting the elderly. Degenerative processes can strike in particular organs or tissues, resulting in damaged joints, such as hips and knees; in weakened bones, as a result of osteoporosis; or as degeneration of the brain, causing severe mental deficits, such as dementia. It has been argued, most notably by the epidemiologist, Thomas McKeown (1912–88), that the main risks to life and good health have occurred in 3 distinct historical phases: accidents and injuries; infections; and finally degenerative diseases of longevity, which can include diseases such as Alzheimer's disease, Parkinson's diseases and some cancers.

One of the best known degenerative diseases is Alzheimer's, first described in 1906 by Alois Alzheimer, but then recognized as only a very rare brain disorder associated with cognitive dysfunction. This type of dementia is now the most common acquired progressive brain syndrome, although its cause remains unknown. Recent figures from the US have shown that Alzheimer's affects more than 4% of the over-60s population, whilst prevalence grows to 20% of the over-80s age group. The impact of chronic degenerative disease is felt not only by individuals and families, but also by social welfare and health care systems.

Cancer

Cancer is caused by a breakdown in the normal processes of cell division and multiplication, resulting in uncontrolled cell growth producing a tumour. In the industrialized world, at the beginning of the twenty-first century, it is estimated that one-third of the population will develop cancer, with the probability currently increasing. This is partly because it is predominantly a disease of middle and old age, and as life expectancy has increased, so too has the incidence of cancer. Several cancers are known to have a genetic basis, and also the environmental impact of some pollutants, known as carcinogens, is becoming increasingly well understood.

Iatrogenic diseases

These diseases arise from medical treatment for another condition. Sometimes the problem may be due to recognized undesirable side-effects of therapeutic drugs, or to an unusual, idiosyncratic reaction to a medicament. A scheme of reporting adverse drug reactions, the so-called ‘yellow card scheme’ was introduced in Britain in the early 1970s, in an attempt to identify such reactions. Surgical procedures, when mishaps or infections result, can also inadvertently cause further disease.

E. M. Tansey

Bibliography

Kiple, K. F. (ed.) (1993). The Cambridge world history of human disease. Cambridge University Press.
McKeown, T. (1979). The role of medicine: dream, mirage or nemesis. Oxford University Press.
McKeown, T. (1988). The origins of human disease. Oxford University Press.

See also allergy; drug abuse; environmental toxicology; genetics, human; Islamic medicine; medicine; mind–body interaction; work and the body.