Human beings are robust and adaptable, able to survive in changing environments and to endure physical and psychological stress. The body’s design incorporates systems that renew and repair it continuously and others that protect it from harm. Many trivial injuries or potential illnesses heal by themselves or are controlled before we are aware of them. However, throughout a lifetime, we are exposed to a relentless stream of minor and more serious diseases and injuries with a variety of effects on the body.
You become ill when something disrupts the normal healthy working of your body. Why you become ill is a question with multiple answers, many of which focus on your genes. Some rare diseases are caused by an inherited faulty gene, but genes are a contributing factor in many other illnesses. In particular, they predict to some extent your chance of developing major diseases of adult life, such as stroke and some types of cancer. Genes also help to determine your susceptibility to many mental health problems, such as schizophrenia and depression. As well as genes, your age, gender, lifestyle, and the environment you live in, are all factors affecting your risk of illness.
New drugs, immunizations, together with advances in hygiene and public sanitation have all contributed to a reduction in mortality from infectious diseases in the developed world, although such diseases remain a major threat to populations in the developing world. Today, major causes of death in the UK are heart disease, cancer, stroke, and accidents. These are strongly associated with lifestyle, and the risk of each can be greatly reduced by making changes in behaviour. As a result, the emphasis in medicine has changed: doctors now recognize that prevention is as important as treatment, and people are informed about how to adapt their lifestyle to stay healthy.
Understanding the body
Understanding disease is easier if you already have some appreciation of the normal structure and function of your body and the way in which its various components are organized.
The body can be divided into a number of major systems that carry out vital functions. For example, the respiratory system enables you to breathe, and the immune system protects you from infection. The bones, muscles, nerves, skin, blood, and other tissues that make up body systems are made of billions of connected cells. Each cell is a specialized, fully functioning unit, and all of its activities are controlled by the genetic code contained in the DNA (deoxyribonucleic acid), which sits within its nucleus.
In this guide, diseases and disorders are mainly grouped in sections under the body system that they affect. Each section begins with a description of the normal anatomy and physiology of a body system to help you to understand the disorders that follow. When you have a problem, you may be referred to a doctor who specializes in disorders of that body system or organ. For example, you may see a dermatologist for a skin complaint and a gastroenterologist for digestive system disorders. However, diseases can also be categorized according to the mechanisms by which they damage the body.
How disease affects the body
The different ways in which diseases damage the body are called disease processes. Several body systems may be damaged by the same process. For example, the major cause of disability and death in the UK, coronary artery disease, comes under the heading of ischaemic disease. This term applies to all diseases in which there are changes in blood vessels, such as a build-up of fatty deposits, that restrict blood flow and starve organs and tissues of oxygen-carrying blood, leaving them unable to function properly.
Similarly cancer, a leading cause of death in the UK, is not a single disease but a group of disorders. Cancerous tumours can affect many different organs and tissues, causing different symptoms, but all cancers consist of cells that reproduce uncontrollably, invade healthy tissue, and may spread to other sites.
Infections occur when microscopic organisms, such as bacteria, invade the body, and range from minor complaints, like boils, to major infectious diseases, such as meningitis.
Metabolic disorders affect chemical processes in the body and are often caused by a failure to produce a certain enzyme or by malfunction of a hormone-producing gland. For example, in one type of diabetes mellitus, normal blood sugar levels cannot be maintained because pancreatic cells produce insufficient amounts of the hormone insulin.
A number of neurological illnesses and mental health problems are caused by disorders of brain chemistry. Diseases such as Alzheimer’s disease and depression are associated with imbalances in the levels of neurotransmitters, chemicals that transmit nerve signals. However, no chemical or structural factor has yet been found for many other mental health problems. In autoimmune disorders, the immune system, which normally protects the body from infections and cancer, attacks the body’s own tissues, disrupting the function of an organ or gland. For example, in rheumatoid arthritis, the immune system attacks and damages the linings of joints, causing pain and sometimes disability.
Although inheritance is increasingly being found to play a part in many major diseases, there is a group of several thousand rare genetic disorders caused solely by a faulty gene inherited from one or both parents. An example is cystic fibrosis, in which a faulty gene causes abnormally thick mucus to be produced in the lungs and digestive tract, resulting in the destruction of lung tissue and reduced absorption of food. In degenerative disorders, the structure and function of tissues and organs are gradually impaired by the loss of specialized cells or tissues, as in osteoarthritis, a wearing away of the smooth cartilage covering joint surfaces. Though traditionally associated with aging, a growing number of these diseases, such as cataract (loss of transparency in the lens of the eye), are also caused by exposure to strong sunlight, toxins, and prolonged use of certain drugs.
Injury covers all types of deliberate and accidental damage to the body. Every year in the UK, there are about 17,000 fatal injuries; the major causes are falls, traffic accidents, intentional self-harm, and accidental poisoning.
Your susceptibility to disease
Some contributory factors for developing illness, such as your genes, gender, ethnicity, and age are largely unalterable. However, you can reduce your risk of ill health by following the guidelines for healthy living given in this guide (see Taking control of your health).
Many illnesses, such as psychological disorders, can occur at any age. However, particular age groups are vulnerable to certain problems. Babies are susceptible to infectious diseases because their immune system is not fully developed, and young children tend to have frequent accidents while their physical skills, coordination and balance, are still developing. People in their teens and in early adulthood are more likely to injure themselves. For example, most disability and death in young men is associated with risk-taking behaviour involving vehicles and weapons. Adolescents are prone to eating disorders, depression, and substance abuse. Young people who eat unhealthily, take too little exercise, smoke, and drink too much alcohol face a future risk of major diseases, such as heart disease, cancer, and stroke, that are increasingly common from middle age onwards. The incidence of long-term illness and disability increases with age and, for some people, poor physical health can lead to mental health problems. Susceptibility to disease is closely linked to social factors such as poverty. For example, in the UK, rates of heart disease are higher in poor families than in those with a reasonable standard of living.
Changing patterns of disease
During the past 50 years, many major infectious diseases have been brought under control in the developed world, and smallpox, one of the oldest diseases of humanity, has been eradicated globally. However, AIDS, a relatively new disease caused by HIV infection, is a major cause of death. In addition, tuberculosis (TB) has become harder to control, because some strains have become resistant to antibiotics and due to reduced immunity in HIV-infected people.
For most of the 20th century, cardiovascular disease, cancer, and stroke were major causes of death in the developed world. They are now also becoming common in developing societies due to factors associated with affluence, such as a high-fat diet. The high incidence of these diseases is also due in part to the increasing age of the population. As the number of people over 65 is predicted to double by the year 2025, these diseases are likely to continue to be major global threats to health.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.
The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.