Biology AS: Causes of Disease

Disease: - A malfunction of body or mind which has an adverse affect of good health.

- It has mental, physical and social aspects.

For a microorganism to be considered a pathogen it must:

- Gain entry to the host

- Colonise the tissues of the host

- Resist the defences of the host

- Cause damage to the host tissues

Colonisation of tissues results in infection.

Disease occurs when an infection develops into recognisable symptoms.

When a pathogen moves from one organism to another it is called transmission.

Pathogens enter an organism by penetrating one of its interfaces.

Interface: Surface or boundary linking two systems (e.g. external and internal environment)

Some interfaces have evolved to allow exchange across the link.

As a result the body linings at these points are thin, moist (and sticky), large surface area and are well supplied with blood vessels.

Interfaces are therefore a common point of entry and include: 

1.  Gas exchange system: Pathogens that cause influenza, TB and bronchitis infect in this way.

2. Digestive system: Food and water may carry pathogens. Cholera, typhoid and dysentery pathogens enter the body in this way.

To help prevent the entry of pathogens, the body has a number of natural defences:

1. Mucus layer: Covers exchange surfaces and makes them hard to penetrate.

2. Production of enzymes: These break down pathogens that enter the body.

3. Production of stomach acid: This kills microorganisms.

Pathogens cause disease by:

1. Damaging host tissues: Viruses inhibit the synthesis of DNA, RNA and proteins.

Many pathogens break down the cell membranes.

2. Producing toxins: e.g.Cholera bacterium-produces a toxin that causes water loss.

Correlation: When a change in one of two variables is reflected by a change in the other variable.

Causal link: When it is proven and there is evidence that the change in one caused the change in the other.

Looking at data critically:

1. Has the right factor been measured and have the right questions been asked?

2. How were the data collected- were the methods reliable and was the right apparatus used? (Accuracy)

3. Do those collecting data have a vested interest in the outcome? (Bias)

4. Has the study been repeated, with the same results and conclusion, by other people? (Reliability)

Risk: A measure of the probability that damage to health will occur as a result of a given hazard.

The concept of risk has two elements:

1. The probability that a hazardous event will occur.

2. The consequences of that hazardous event.

Measurement of risk: Percentage from 0% (no harm will occur) to 100% (harm will certainly occur.

Risk is often relative, it is measured by comparing the likelihood of harm occuring in those exposed to the hazard with those who are not exposed to it.

Misleading statistics: Reports in the media often focus on a single figure and are therefore misleading.

Our lifestyle can expose us to environmental and carcinogenic factors that put us at risk of contracting cancer.

Specific lifestyle factors that contribute to cancer include:

1. Smoking: Smokers and passive smokers.

2. Diet: Strong evidence that a low-fat, high-fibre diet, rich in fruit and vegetables, reduces risk.

3. Obesity: Being overweight increases the risk of cancer.

4. Physical activity: People who take regular exercise are at lower risk than someone who does not.

5. Sunlight: The more someone is exposed to sunlight or light from sunbeds, the greater is the risk of skin cancer.

Lifestyle factors we can control that affect the risk of CHD are:

1. Smoking: Smokers are between 2 and 6 times more likely to suffer from CHD.

2. High blood pressure: Excessive prolonged stress, certain diets, and lack of exercise all increase BP and hence the risk of CHD.

3. Blood cholesterol levels: These can be kept lower by including fewer saturated fatty acids in the diet.

4. Obesity: BMI of 25+ brings a increased risk of CHD.

5. Diet: High levels of salt raise BP. High levels of FAs increase blood cholesterol. Both therefore increase risk of CHD. Dietary fibre lowers blood cholesterol levels.

6. Physical acivity: Aerobic exercise can lower BP and blood cholesterol.