Chapter 2 Characteristics, classification and incidence of disease
WHAT IS DISEASE?
Limits of normality
Most quantifiable biological characteristics are normally distributed, in statistical terms, about an average value. There are no constant numbers that can be used to define a normal height, weight, serum sodium concentration, etc. Normality, when quantifiable, is expressed as a normal range, usually encompassed by two standard deviations (for a ‘normally’ distributed feature) either side of the mean (Ch. 4). The probability that a measurable characteristic is abnormal increases the nearer it is to the limits of the normal range, but a value lying outside the normal range is not necessarily indicative of abnormality—it is just very probably abnormal.
A distinction must also be drawn between what is usual and what is normal. It is usual to find atheroma (Ch. 13) in an elderly individual—but is it normal? In contrast, atheroma in a teenager is so unusual that it would be regarded as abnormal and worthy of further investigation.
Responses to the environment
Disease: failure of adaptation
Darwinian medicine
Ageing and adaptation
One of the main features of ageing is progressive inability of the individual to deal with new or worsening environmental threats (Ch. 12). This is exemplified by the gradual impairment of immune responses, resulting in:
CHARACTERISTICS OF DISEASE
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All diseases have a set of characteristic features enabling them to be better understood, categorised and diagnosed. For many diseases, however, our knowledge is still incomplete or subject to controversy. The characteristics of any disease are (Fig. 2.1):
The aetiology and pathogenesis of a disease may be combined as aetiopathogenesis.
Aetiology
General categories of aetiological agents include:
Identification of the causes of disease
In terms of causation, diseases may be:
Most common diseases have an entirely environmental cause, but genetic influences in disease susceptibility are being increasingly discovered, and many diseases with no previously known cause are being shown to be due to genetic abnormalities (Ch. 3). This is the reward of applying the principles of clinical genetics and the new techniques of molecular biology to the study of human disease. The extent to which a disease is due to genetic or environmental causes can often be deduced from some of its main features or its association with host factors.
Probability of disease
The relationship between the quantity of causal agent and the probability that disease will result is not always simply linear (Fig. 2.2). For example, many infections occur only on exposure to a sufficient dose of micro-organisms; the body’s defence mechanisms have to be overcome before disease results. Some agents capable of causing disease, such as alcohol, are actually beneficial in small doses; those who abstain from alcohol have a slightly higher risk of premature death from ischaemic heart disease.
Host predisposition to disease
Some diseases occur more commonly in individuals with a congenital predisposition. For example, ankylosing spondylitis (Ch. 25), a disabling inflammatory disease of the spinal joints of unknown aetiology, occurs more commonly in individuals with the HLA-B27 allele.
Some diseases predispose patients to the risk of developing other diseases. Diseases associated with an increased risk of cancer are designated premalignant conditions; for example, hepatic cirrhosis predisposes to hepatocellular carcinoma, and ulcerative colitis predisposes to carcinoma of the large intestine. The histologically identifiable antecedent lesion from which the cancers directly develop is designated the premalignant lesion.
Some diseases predispose to others because they have a permissive effect, allowing environmental agents that are not normally pathogenic to cause disease. This is exemplified by opportunistic infections in patients with impaired defence mechanisms resulting in infection by organisms not normally harmful (i.e. non-pathogenic) to humans (Ch. 9). Patients with leukaemia or the acquired immune deficiency syndrome (AIDS), organ transplant recipients, or other patients treated with cytotoxic drugs or steroids, are susceptible to infections such as pneumonia due to Aspergillus fungi, cytomegalovirus or Pneumocystis jiroveci.
Causal associations
Causal associations may be neither exclusive nor absolute. For example, because some heavy cigarette smokers never develop lung cancer, smoking cannot alone be regarded as a sufficient cause; other factors are required. Conversely, because some non-smokers develop lung cancer, smoking cannot be regarded as a necessary cause; other causative factors must exist.
Koch’s postulates
Pathogenesis
Examples of pathogeneses of disease include:
Structural and functional manifestations
Structural abnormalities
Common structural abnormalities causing ill health are:
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