Biology Core- Infectious Diseases

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Causes of Disease

A disease occurs when the normal functioning of the body is disturbed. Infectious diseases can be passed on from one person to another but non-infectious diseases cannot.

Organisms that cause infectious diseases are called pathogens.

A number of different types of organisms can be pathogens...

  • Some bacteria
  • Some single celled animals
  • Some worms
  • Viruses
  • Some Fungi

Pathogens may reproduce rapidly in the body, either damaging cells directly or producing chemicals called toxins, which make us feel ill.

Viruses damage cells by taking over the cell and reproducing inside them.

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Type of disease                                Description                                 Examples

Body Disorder          Incorrect fuction of a particular organ      Diabetes, Cancer

Deficiency Disease               Lack of a mineral or vitamin         Anaemia, Scurvy

Genetic Disease                Caused by a defective gene            Colour blindness

Infectious Disease              Caused by a pathogen                            TB

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How the body protects itself

The skin covers most of the body and is quite good at stopping pathogens entering the body. The body has a number of other defences that it uses in order to try and stop pathogens entering.

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If the pathogens do enter the body when the body will attack them in a number of ways.

The area that is infected will often become inflamed and two types of white blood cells attack the pathogen.

The pathogen is detected by the white blood cells because it has foreign chemical groups called antigens on its surface. The antibodies that are produced by the body are specific to a particular pathogen or toxin and will only destroy that particular antigen.

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Sometimes a pathogen can make us ill before out bodies immune system can destroy it, We may sometimes need to take a drug called antibiotics to kill the pathogen,

  • Antibiotics are chemicals that are usually produced by microorganisms that kill bacteria and fungi, They do not have any effect on viruses.

The first antibiotic to be widely used was penicillin. Today there are a number of different antibiotics that are used to treat different bacteria. This has meant that some diseases that once killed millions of people can now be treated.

There is a problem, however. More and more strains of bacteria are appearing that are resistant to antibiotics.

A genetic change, or mutation, in the bacteria population can enable large population of resistant bacteria to appear.

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This process has occurred in many different types of bacteria, including the TB causing bacterium and one called MRSA. These bacteria are now resistant to many different types of antibiotics and so are very difficult to treat.

There are various ways that doctors try to prevent the spread of these resistant bacteria:

- "I tell my patients to finish the dose of antibiotics even if they feel better"

- "I change the antibiotics that i prescribe regularly and sometimes use combinations of a different antibiotic"

- "I prescribe antibiotics only in serious cases caused by bacteria"

- "I always wash my hands with antiseptic between patients"

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One important weapon against resistant bacteria is the use of antiseptics.

  • Antiseptics are artificial chemicals that kill pathogens outside the body.

They were first used by an Austrian doctor called Dr Semmelweis to sterilise medical instruments. The use of antiseptics in hospitals is vital in preventing the spread of resistant bacteria,

  • An antiseptic is usually used on the body and a disinfectant is usually used on other surfaces.

Dr Semmelweis

 In the 19th century, Ignaz Semmelweis realised the importance of cleanliness in hospitals. However, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens that could be killed.

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When our body encounters a pathogen, white blood cells make antibodies against the pathogen. If they encounter the same pathogen again in the future then antibodies are produced faster and the pathogen is killed more quickly.

This is called Immunity. This idea has been used in vaccinations.

  • A vaccine contains harmless versions of the pathogen which stimulate immunity.

A vaccination stimulates our body to make antibodies and special cells called memory cells. When the real pathogen comes along, it will be destroyed quickly,

This type of immunity where the antibodies are made by the person is call active immunity.

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Sometimes it might be too late to give somebody this type of vaccination because they already have the pathogen. They can be given an injection containing antibodies made by another person or animal.

This is called passive immunity. A similar thing happens when a baby receives antibodies from its mother across the placenta.

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How long do vaccines protect us for?

Some vaccines protect us for a long time because the memory cells survive for many years. The problem is that some diseases such as influenza need new vaccinations ever year.

This is because the virus mutates and changes the shape of its outer coat. This means that different antibodies are needed, so a different vaccinations is required.

The human immunodeficiency virus (HIV) also mutates regularly and weakens the immune system. This is making it very difficult to produce a vaccine.

For a vaccine to reduce or completly get rid of a disease, most of the population must be treated. This has sometimes proved difficult as some people are worried about the side effects of certiain vacciones.

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Drugs and Health

  • Drugs are chemicals that alter the functioning of the body

Some drugs such as antibiotics are often beneficial to our body if used correctly. Others can be harmful,particularly those that are used recreationally.

Many drugs addictive.This means that people want to carry on using them even though they may be having harmful effects. If they stop taking them they may suffer from unpleasant side effects called withdrawal symptoms. It also means that people develop tolerance to the drug, which means that they need to take a bigger doses to have the same effect. Heroin and cocaine are very addictive.

In order to control drugs, many can only be bought with a prescription. Illegal drugs are classified into groups. Class A drugs are the most dangerous, and Class C are the least dangerous. If people are caught with illegal Class A drugs the penalties are the highest.

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-Sedatives/Depressants- slow down the action of the brain, e.g. barbiturates,alcohol,solvents and temazapam.

-Stimulants- increase the activity of the brain, e.g. nicotine, ecstasy and caffeine.

-Pain Killers- block nerve impulses and so reduce pain, e.g. asprin and heroin.

-Performance Enhancers- cause muscle growth, e.g. anabolic steriods.

-Hallucinogens- distort what is seen or heard, e.g. cannabis, LSD

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Many people cannot give up smoking tabacco because it contains the drug nicotine. This is addictive. The nicotine is harmful to the body but most damage is done by the other chemicals in the tabacco smoke.

  • Chemicals in the tar may cause cells in the lung to divide uncontrollably. This can cause lung cancer.
  • The mucus collects in the alveoli and may become infexted. This may lead to the walls od the alveoli being damaged. This reduces gaseous exchange and is called emphysema.
  • The heat and chemicals in the smoke destroy the cilia on the cells lining and the airways. The goblet cells also produce more mucus than normal. The bronchioles infected. This is called bronchitis.
  • The nicotine can cause an increase in the blood pressure increasing the chances of a heart attack.
Smoking tabacco is particularly dangerous for pregnant women. The carbon monoxide in the smoke combines with oxygen in the mother's blood and this can deprove the baby of oxygen. This may lead to low birth weight.
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Drinking Alcohol

Drinking alcohol can have a number of effects on the body:

Short term effects-

  • upsets balance and muscle control
  • blurred vision and speech
  • slower reactions
  • helps people relax

Long term effects-

  • damage to the liver (cirrhosis)
  • brain damage

Owing to the effects of alcohol on the body there is a legal limit for the level of alcohol in the blood of drivers and pilots.

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Diseases of excess

It is important to maintain a balanced diet for the healthy functioning of the body. In the develped world many people eat too much food. This can make a person more likely to get various diseases.


If a person eats food faster that it is used up by the body then the excess will be stored. Much of this will be stored as fat and can lead to obesity.

Obesity can be linked to a number of different health risks:

  • Arthritis
  • Diabetes
  • Breat Cancer
  • High Blood Pressure
  • Heart Dissease

It is possible to estimate if a person is underweight, normal, overweight or obese by using the formula: BMI- Body Mass Index

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Blood Pressure

Contractions of the heart pump blood out into the arteries under pressure. This is so it can reach all parts of the body,

Doctore often measure the blood pressure in the arteries and give two figures for example 120 over 80. The highest figure is called the systolic pressure and is the pressure when the heart contracts. The second figure is when the heart is relaxed and is called the diastolic pressure.

Blood pressure varies depending on various factors. The following factors can increase blood pressure:

  • High Salt and fat in the diet
  • Lack of exercise
  • High alcohol intake
  • Stress
  • Obesity
  • Ageing
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Heart Disease

The heart is made up of muscle cells that need to contract throughout life. This needs a steady supply of energy so the cells need oxygen and glucose at all times for respiration. This is supplied by blood vessels.

Fatty deposits can form in these blood vessels and reduce the flow of oxygen and glucose to the heart muscle cells.

  • This reduction in blood flow causes heart disease and if an area of muscle stops beating then this is a heart attack.

There are many factors that can make it more likely for a person to have heart disease: Cholesterol is an important chemical in the body. It is made in the liver and carried in the blood. In the blood it is carried by chemicals called lipoproteins. There are 2 types called HDL's and LDL's. The balance of these is important as LDL's can increase the chance of having a heart attack and HDL's can protect the heart. Saturated fats increase LDL's and so are bad for you, but polyunsaturated fats increase HDL's.

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Deficiency Diseases

In the developing world many people cannot get a balanced diet because there is not enough food to eat.

  • Eating too little of one type of food substance can lead to a deficiency disease.

Examples of deficiency diseases are:

  • Anameia- due to lack of iron
  • Scurvy- due to lack of vitamin C
  • Kwashiorkor- due to lack of protein

There are times when people do not eat enough food although there is food available. They put themselves on a diet because they have a poor self-image or think that they are overweight when they are not. This can reduce their resistance to infection and cause irregular periods in women. It may lead to illnesses such as anorexia.

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