Non-communicable diseases

1. A non-communicable disease is not caused by a pathogen, so it cannot be passed from one person to another.
2. Most non-communicable diseases develop slowly over many years and last for a long time once they appear.
3. Very few of them have a single cause, because most arise from the interaction of several factors acting on the body at the same time.
4. These factors fall into three groups.
   1. Genetic factors are the alleles a person inherits, which can make them more or less likely to develop a disease.
   2. Lifestyle factors are the choices a person makes, such as their diet, how much exercise they take, whether they smoke and how much alcohol they drink.
   3. Environmental factors are features of a person's surroundings, such as air pollution, harmful chemicals breathed in at work or ultraviolet light from the Sun.
5. Each of these factors is called a risk factor, because it raises the probability of developing a disease without making the disease certain.
6. A person with no obvious risk factors can still develop a non-communicable disease, and a person with several risk factors may never develop one.
7. When risk factors combine, the overall risk usually rises by more than the risk from each factor on its own, because the factors often damage the same organ in different ways.
8. The main non-communicable diseases caused in this way are cardiovascular diseases, many forms of cancer, lung diseases such as bronchitis, liver diseases such as cirrhosis and diseases influenced by nutrition, including type 2 diabetes.

Cardiovascular disease

1. Cardiovascular disease includes coronary heart disease, heart attacks, strokes and high blood pressure.
2. In coronary heart disease, fatty deposits containing cholesterol build up in the inner walls of the coronary arteries, which supply the heart muscle with blood.
3. The deposits narrow the lumen of each artery, so less blood reaches the heart muscle.
4. With less blood, the heart muscle receives less oxygen and glucose, so it cannot carry out enough aerobic respiration to release the energy it needs to contract.
5. If a fatty deposit breaks open, a blood clot can form on it and block the artery completely, and the heart muscle beyond the blockage dies in a heart attack.
6. A clot that blocks an artery supplying the brain causes a stroke.
7. Several risk factors interact to cause cardiovascular disease.
   1. A diet high in saturated fat raises the concentration of cholesterol in the blood, which speeds up the build-up of fatty deposits.
   2. A diet high in salt raises blood pressure, and high blood pressure damages the lining of the arteries so that deposits form more easily.
   3. Smoking damages the lining of the arteries, and the carbon monoxide in the smoke reduces the amount of oxygen the blood can carry, so the heart has to work harder.
   4. The nicotine in tobacco raises the heart rate and blood pressure.
   5. Drinking large amounts of alcohol over many years raises blood pressure.
   6. Lack of exercise and obesity raise blood pressure and blood cholesterol and put extra strain on the heart.
   7. Some people inherit alleles that make their liver produce more cholesterol, so they develop fatty deposits even when they eat a healthy diet.
8. A smoker with high blood pressure and a high-fat diet has a far greater risk than a person with only one of these factors, because each factor adds to the damage in the same arteries.

Cancer

1. Many forms of cancer are caused by the interaction of several factors rather than by a single cause.
2. Smoking is the main risk factor for lung cancer, because tobacco smoke contains chemicals that damage the DNA of the cells lining the airways.
3. Exposure to ultraviolet light from the Sun or from sunbeds increases the risk of skin cancer.
4. Heavy alcohol consumption and obesity both increase the risk of several cancers, including cancers of the mouth, liver, bowel and breast.
5. Some people inherit faulty alleles of genes such as BRCA1 and BRCA2, which greatly increase their risk of breast cancer and ovarian cancer.
6. A person who inherits one of these alleles and also smokes or drinks heavily has a higher risk still, which shows how genetic and lifestyle factors combine.

Bronchitis

1. Bronchitis affects the bronchi, the tubes that carry air into the lungs.
2. The inflamed lining of the bronchi produces extra mucus, and the cilia that normally sweep mucus out of the airways are damaged, so mucus collects in the lungs.
3. The person develops a persistent cough, becomes short of breath and is more likely to get lung infections.
4. When the condition lasts for months or years, it is called chronic bronchitis.
5. The main risk factor is smoking, because the tar and other chemicals in smoke irritate the airways and stop the cilia from beating.
6. Air pollution, including smoke, dust and fumes breathed in at work, also irritates the airways and increases the risk.
7. Inherited differences in how the lungs respond to irritants help to explain why some smokers develop bronchitis while others do not.
8. A smoker who also lives or works in polluted air has a much higher risk than a person exposed to only one of these factors.

Cirrhosis

1. In cirrhosis, healthy liver tissue is gradually replaced by scar tissue.
2. Scar tissue cannot carry out the jobs of the liver, such as breaking down toxins and making important proteins, so the liver slowly fails.
3. The main risk factor is drinking large amounts of alcohol over many years, because liver cells are damaged each time they break down alcohol.
4. Obesity and a diet high in fat and sugar cause fat to build up in the liver, and this can also lead to scarring.
5. Long-term infection with the hepatitis B or hepatitis C virus inflames the liver, so a person who is infected and also drinks heavily is at particularly high risk.
6. Cirrhosis often causes no symptoms until a large part of the liver has been damaged.

Type 2 diabetes

1. Type 2 diabetes is a disease strongly influenced by nutrition.
2. In type 2 diabetes, body cells stop responding properly to the hormone insulin, so blood glucose concentration stays too high after a meal.
3. The strongest risk factor is obesity, especially fat stored around the abdomen.
4. A diet high in sugar and fat, combined with little exercise, leads to weight gain and so increases the risk.
5. The risk also rises with age and is higher in people who have a close relative with the disease, which shows that inherited alleles play a part.
6. Losing weight, eating a balanced diet and taking regular exercise greatly reduce the risk, even for someone with a family history of the disease.