Animals and humans are comprised of a number of exchange systems in which substances are exchanged with the environment, like the gas system or the digestive system. The body also needs to be able to transport substances from one part to another and it achieves that with the blood system. In a healthy body each system is able to function effectively. However, diseases can prevent a system from functioning properly. Some are caused by microorganisms, like tuberculosis or cholera, while others are non-communicable and can affect the lungs or even the heart. When you get ill your blood contains numerous defensive functions and drugs, like antibiotics, can also help to prevent a disease spreading.

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Lifestyle

A person’s lifestyle also has a big impact on their health. For example, how much they exercise they do or how healthily they eat.

Different diseases carry with them different risk factors, as you can see from the table below:

Some risk factors, like age, are beyond someone’s control while others are not. Scientists have shown links between an unhealthy lifestyle and diseases like coronary heart disease and cancer. However, if someone at risk changes their lifestyle for the better then the risk of contracting these conditions can be reduced.

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Correlation and Causation

Risk factors associated with disease can be discovered through epidemiology. Epidemiology looks at the distribution, associations and incidence of different diseases with the aim to find out their causes and so assist with finding preventative measures.

Correlation, or association, is where two variables act in the same way. On a graph this would be seen as one variable increasing as the other increases (positive correlation) or one variable decreasing as the other decreases (negative correlation). However, this doesn’t mean that one variable causes the other to act like that (causation). There might be another factor involved or it could even just be coincidental.

To find out if a relationship is causal experiments need to be carried out to look into the relationship. This way mechanisms explaining the correlation can prove that one variable does affect the other.

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Causes of disease

A disease is some form of disorder within the body. Two main causes of disease:

• pathogens
• lifestyle

Pathogens

Pathogens can be:

• bacteria
• viruses
• fungi

They spread infectious disease and can enter the body by penetrating any of the systems or interfaces in the body which exchange with the environment, like the gas-exchange or digestive systems.

• First a pathogen must be transmitted from its current human host. It can achieve this in a number of ways, like through drinking water, by direct contact, or via aerosol droplets.
• The pathogen must now enter its next victim. It can’t diffuse through the thick skin so it uses other routes, for example through a cut or by using an exchange surface, like the lungs.
• The human body has a number of very effective defence mechanisms against pathogens. However, if only a few get through these then they can multiply in numbers very rapidly, causing disease.
• A pathogen can then harm their in two main ways host:
  • First of all they reproduce in the host cells. This uses up the cell’s resources and prevents it from functioning properly. Usually the pathogens then burst out of the cell thereby killing it.
  • They also produce toxins. These can interfere with the function of the body in a number of ways including inhibiting enzymes, interfering with synapses, and causing DNA mutations.