When a person has a disease, a part of that person’s body fails to function properly. Diseases produce different patterns of symptoms: knowing these helps a doctor to recognize many different diseases. Three of the most common categories of disease are HEART DISEASE , INFECTIOUS DISEASES, and CANCER. Various factors can affect the risk of a disease developing; for example, smoking is known to increase the risk of many diseases, whereas regular exercise may reduce the risk.
Table 26. SOME OTHER CATEGORIES OF DISEASE
|CATEGORY||CAUSE AND EXAMPLES|
|Nutritional deficiencies||Lack of an essential nutrient in the diet Example: scurvy, caused by lack of vitamin C|
|Genetic disorders||Inheritance of faulty genes Examples: cystic fibrosis, haemophilia|
|Degenerative diseases||Gradual loss of function in body parts Example: osteoarthritis|
|Endocrine disorders||Disturbance of the body’s hormonal balance Example: diabetes mellitus|
|Autoimmune diseases||Immune system attacks the body’s own tissues Example: rheumatoid arthritis|
|Allergies||Immune system’s sensitivity to substances from outside the body Example: hay fever|
Heart disease is caused by narrowing of the coronary arteries, the vessels that supply blood to the heart muscle. Some factors that increase the risk of getting the disease are smoking, being overweight, a diet rich in animal fats, and a lack of exercise. Heart disease is common in rich countries. Drugs, and in some cases surgery, are used to treat it.
The narrowing of the coronary arteries in heart disease is caused by fatty deposits, called atheroma, on the walls of the arteries. Atheroma can also develop in arteries supplying the brain. This can lead to a stroke, in which the blood supply to an area of brain is cut off.
A high concentration of particles like this in the blood is thought to increase the risk of atheroma. Called low-density lipoprotein particles, they are composed of fatty substances and some protein. The level of these particles in the blood tends to be higher if a person’s diet is rich in fats of animal origin.
Infectious diseases are caused by organisms that invade body tissues or organs and affect their functioning. Infections can be passed between people and can affect any part of the body. The two main types of infectious organisms are bacteria and viruses. Other types include fungi, protozoa, and worms. Some infectious diseases can be treated with drugs.
Bacteria are simple, single-celled organisms. Some cause problems by releasing damaging substances called toxins. The bacteria seen here cause meningitis, a dangerous inflammation of the coverings of the brain and spinal cord. Other bacterial diseases include cholera, typhoid, bacterial pneumonia, and tuberculosis.
Viruses are the smallest infectious particles. They need to invade other living cells to replicate. The human immunodeficiency virus (HIV) infects certain white blood cells. Eventually, an infected person is likely to develop AIDS, a disease in which the body’s immune system is severely weakened. Viral diseases include colds, flu, measles, and mumps.
In cancer, there is rapid and uncontrolled growth of body cells. These cells are often grouped together as a lump, called a tumour. Common sites include the lungs, large intestine, and breasts. The tumour may spread to surrounding tissues and later to other parts of the body. In some cases, cancer is fatal. However, the various treatment choices mean that it can often be cured.