Disease

I

Introduction

Disease, any state in which the health of the human organism is impaired. All diseases involve a breakdown of the body's natural defence systems or those regulating the internal environment. Even when a cause is not known, a disease can almost always be understood in terms of the physiological or mental processes that are disrupted.

II

Symptoms

Understanding of diseases depends on a clear delineation of the symptoms, which are manifestations of altered life processes. Symptoms may range from subjective reports of pain, such as headache or backache, to visible conditions, such as a swelling or a rash. Common symptoms include changes in body temperature (such as fever), fatigue, loss or gain of weight, and pain or tenderness of muscles or internal organs. Further examination may reveal: the presence of infectious organisms, as detected by growth in special nutrient solutions; broken bones seen during radiological examination; changes in the composition of blood cells; and cancerous growth identified microscopically on surgically removed tissue.

With increasing use of laboratory tests in routine physical examinations of apparently healthy people, doctors are more often finding diseases without symptoms noticeable to the patient. For example, high blood pressure can be detected in its early stages before it does measurable damage to the heart and blood vessels. Another condition, mitral-valve prolapse, in which one of the heart valves functions abnormally, occurs in about 10 per cent of all people examined but usually causes no symptoms. Apparently most people with this condition are healthy, but some may have an increased risk of autoimmune disease. The development and widespread use of more sensitive tests may require careful use of the term disease.

III

Classification

Diseases can be classified by their: site (bone, heart, liver); natural history (acute or chronic); course (progressive or intermittent); or by other schemes. Two of the most useful ways to classify are by cause and by which biological process is disturbed.

A

Classification by Cause

The classification-by-cause approach arose from the study of infectious diseases, which were among the first illnesses to be understood. Examples include: pneumonia, cholera, and gonorrhoea, caused by bacteria; and smallpox, measles, and hepatitis, caused by viruses. Even when a causative organism is identified, however, infection is not the only important part of the disease process. Many people may be exposed to the influenza virus, for example, but some stay well, others become moderately ill, and weak or old people may die. The state of the immune system and the overall health of the person influence the disease course.

Non-infectious causes of disease are also known, many of them occupational. For example, long-term exposure to coal dust predisposes miners to a respiratory disease called black lung, and exposure to cotton dust similarly predisposes cotton workers to brown lung. Here again, exposure to the mine or cotton dust is not the only cause of the disease. In both cases workers who smoke cigarettes are known to have a higher chance of developing impaired respiratory function. The same is true of the lung disease caused by exposure to asbestos (asbestosis). More recently, occupational exposure to fine powders and some organic chemicals has been found to produce allergy. Scientists looking at occupational and environmental diseases are studying why some workers exposed to these chemicals experience breathing difficulties and severe sneezing, but others in the same work area do not.

A new type of disease cause called “harmful lifestyles” entered the medical vocabulary in the 1970s. Smoking cigarettes, drinking alcohol excessively, overeating or not exercising, and/or living in a state of constant stress are all now considered to predispose one to illness. These are not causes in the traditional sense. The relationship between eating a high-fat diet and having a heart attack, for example, is even more complicated than the process by which infection causes symptoms.