disease

A false-colour micrograph of the Herpes simplex or cold sore virus. Herpes is the name given to any of several infectious diseases - including cold sores, genital herpes, shingles, chickenpox, and glandular fever - caused by the viruses in the herpes group.

Condition that disturbs or impairs the normal state of an organism. Diseases can occur in all living things, and normally affect the functioning of cells, tissues, organs, or systems. Diseases are usually characterized by specific symptoms and signs, and can be mild and short-lasting (such as the common cold) or severe enough to decimate a whole species (such as Dutch elm disease). Diseases can be classified as infectious or noninfectious. Infectious diseases are caused by micro-organisms, such as bacteria and viruses, invading the body; they can be spread across a species, or transmitted between one or more species. All other diseases can be grouped together as noninfectious diseases. These can have many causes: they may be inherited (congenital diseases); they may be caused by the ingestion or absorption of harmful substances, such as toxins; they can result from poor nutrition or hygiene; or they may arise from injury or ageing. The causes of some diseases are still unknown.

Some diseases occur mainly in certain climates or geographical regions of the world. These diseases are said to be endemic in the regions concerned. For example, African sleeping sickness, which is carried by the tsetse fly, is found mainly in the very hot, humid regions of Africa. Similarly, malaria, a disease spread by mosquitoes, is usually found in or near the marsh or stagnant water that provide breeding grounds for the insect. Other diseases may be seasonal - such as influenza, which tends to occur mainly in winter, or intestinal illnesses that result from food contamination in summer.

Some age groups may be more prone to certain diseases, such as measles in children, meningitis in young adults, and coronary heart disease in the elderly. Other diseases may tend to occur only in certain racial types and are usually genetic in origin, such as sickle-cell disease which is found mainly among people of black African descent. Other diseases, such as black lung, or coal-workers' pneumoconiosis, result from occupational hazards; some of the ‘new’ diseases that have appeared in recent years - such as sick building syndrome and legionnaire's disease, result from modern building designs, while the cause of ME (myalgic encephalomyelitis), or chronic fatigue syndrome, is still unknown.

Infectious diseases In humans, infections caused by micro-organisms (pathogens) are the commonest cause of disease. According to a 1990s World Health Report prepared by the World Health Organization, 17 million deaths (one-third of the total number) occur as a result of infectious diseases. Pathogens are parasites that take over some of the body's cells and tissues, using them for their own growth and reproduction. In the process, the cells and tissues are damaged or destroyed resulting in disease of the host body. These pathogens produce diseases ranging from minor skin infections to life-threatening internal disorders.

There are several ways in which pathogens gain access to the body: through the skin, especially through cuts or wounds; through the respiratory system - for instance, cold and influenza viruses are carried through the air in droplets of moisture and are breathed in; in food or water - bacteria causing food poisoning are taken into the alimentary canal in food, poliomyelitis viruses can be transmitted in water; by vectors (organisms that transmit pathogens to host bodies) - such as rabies which is transmitted in the saliva of mammals. (An infected mammal passing on the rabies virus to another is a vector for rabies.)

Infectious diseases can be classified according to the kind of pathogen that causes them. The most common pathogens are bacteria and viruses, but fungi, protozoans, and worms can also cause infectious diseases. A few kinds of fungi live on the human skin, where they cause athlete's foot, ringworm, and other infections. Disease-producing fungi can also cause brain inflammations and a lung disease called histoplasmosis. Disease-producing protozoans are found chiefly in tropical areas, and cause such diseases as amoebic dysentery, an intestinal infection. Worm infections also cause many serious tropical diseases, including elephantiasis, river blindness, and schistosomiasis.

Control of infectious diseases The human body has many natural defences against the entry of pathogens. For instance, blood clotting can seal a cut or wound thus preventing the entry of pathogens through the skin. The body also has chemical barriers against infection, such as tears, which not only wash foreign substances from the eyes, but also contain enzymes that destroy many common pathogens. In addition, the mucous membranes release protective chemicals. The body's own senses of smell and taste can often detect the presence of bacteria in food before harmful quantities are ingested. Any bacteria that reach the stomach may be killed off by the hydrochloric acid in the stomach's digestive juices. Pathogens that manage to penetrate the body's defences will be recognized as foreign and come under attack from antibodies produced by the white cells in the blood.

Modern medicine has produced many vaccines and antibiotics that can assist the body's own defence mechanisms, or take over when they fail. In addition to alleviating symptoms of disease, some vaccines are used as preventative measures - for instance, the immunization of infants against childhood diseases such as measles and whooping cough, and flu vaccines which are given to the elderly and vulnerable at the onset of winter.

Noninfectious diseases This is a broad term that groups together all those disorders not caused by pathogens. It includes ailments caused by the breakdown of tissues and organs, birth defects, poor diet, environmental factors and occupational hazards, and by stress and tension.

Congenital and genetic diseases These are disorders that are present at birth. Congenital diseases develop while the fetus is in the uterus - for instance, the disease may develop from an infection the mother has suffered during pregnancy, such as rubella, which can result in the baby may being born with a heart defect, mental retardation, or other disorders. Other congenital problems man occur if the mother ingests or is exposed to certain drugs or chemicals, or is exposed to radiation during pregnancy. Genetic, or inherited, diseases involve defects that are passed on from one or both parents, such as Down's syndrome, haemophilia, and sickle cell anaemia. Most congenital and genetic diseases are apparent at birth or in early infancy, although in some cases an inherited disease may not produce symptoms until later in life. On average, about 2% of all babies are born with serious diseases.

Degenerative diseases These tend to be long-term disorders which involve the gradual breakdown of tissues and organs. They are more commonly found in adults, particularly the elderly. Common degenerative diseases include cardiovascular diseases and arthritis. Cardiovascular diseases, which affect the heart and blood vessels, include atherosclerosis, high blood pressure, heart attacks, and strokes. Rheumatoid arthritis, the cause of which is unknown, occurs in people of all ages, although it is most common in middle-aged adults. Its effects are pain and swelling in many joints throughout the body, and it can lead to deformity and crippling. Osteoarthritis results from wear and tear on the joints, especially those of the knees, hips, and fingers. It is mainly a disease of older adults, although it can develop in younger people, particularly professional sportspeople. The disease seldom causes crippling, although the pain may severely restrict the sufferer's activities.

Environmental and occupational diseases Many environmental factors, such as air and waste pollution, can produce serious diseases. Air pollution from industrial emissions and vehicle exhausts can irritate the eyes and nose, and also result in respiratory disorders such as emphysema, bronchitis, and other lung diseases. Seepage of factory waste and chemical fertilizers into waterways can pollute drinking water, leading to serious illnesses. Other disorders that are classified as environmental diseases may be self-afflicted, such as by the habitual usage of tobacco, alcohol, and drugs. Overuse of these substances may lead to a range of serious diseases, including heart disease, liver and brain damage, and cancer. Occupational diseases can result from industrial hazards such as exposure to radiation and toxic chemicals, noise pollution from machinery, traffic, and aircraft, or the inhalation and absorption of harmful substances in industries as diverse as agriculture and textiles. These can result in chronic disorders of the body's metabolic and nervous functions and lead to serious long-term, or even terminal, illnesses. The list of environmental and occupational diseases can also include less severe problems, such as RSI (repetitive strain injury), which has become widespread, especially among computer users.

Nutritional deficiency diseases These can be divided into two categories: primary deficiency diseases which are a direct result of an inadequate supply of an essential nutrient; and secondary deficiency diseases which result from the body's failure to make adequate use of an essential nutrient. This can be due either to an inability to absorb the nutrient, or an inability to metabolise the nutrient once it has been absorbed.

In the poorer countries of the developing world primary deficiency diseases are, for the most part, endemic to regions where poverty and famine are common, or where essential nutrients are lacking in the staple diet. For instance, beri-beri - which is a deficiency disease resulting from a lack of thiamine, vitamin B, in the diet - was endemic in the 19th and early 20th centuries in communities where polished rice or highly milled wheat was the staple food. Food fortification has helped to eliminate the disease from more prosperous areas, but it is still common in the more remote and poorer communities. Cretinism occurs in areas in which severe iodine deficiency, and thus endemic goitre, has been prevalent for several generations. Endemic cretinism is known to occur in parts of Nepal, the Andes, the Democratic Republic of Congo (formerly Zaire), and New Guinea, and in some communities may affect 1-5% of the population.

Iron deficiency anaemia, in which the red blood cells contain abnormally low amounts of haemoglobin, occurs in all countries of the world. In the Middle East, Africa and Asia, as much as 20% of the population is affected. It is still common in the developed, particularly in women of childbearing age. Iron deficiency may be due to an iron-poor diet, poor absorption of the mineral due to disease, or an unreplaced loss of iron from menstruation or other bleeding. Pernicious anaemia, which is caused by a deficiency of vitamin B12 is usually a secondary hereditary disease and results from a lack of the intrinsic factor required for absorption of the vitamin. However, it also known to occur in vegans who have lived without B12-containing animal foods for several years, and in these cases it is regarded as a primary deficiency disease.

In the wealthier developed countries of the world, many instances of primary deficiency diseases are self-inflicted and the direct result of weight-loss and slimming regimes, whereby essential nutrients are omitted from the basic diet. When food obsessions are taken to the extreme, as in bulimia nervosa and anorexia nervosa, severe, and even terminal, disorders can be inflicted on an otherwise healthy body.