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KS3 Bitesize



This Revision Bite is about disease - the microbes that cause it and what our bodies do to fight and prevent it.


This Revision Bite covers:


Many living things are so small that they can only be seen through a microscope. These living things are called microorganisms or microbes. There are three main types of microbe:

  • fungi

  • bacteria

  • viruses


Diagram of a yeast cell showing the cell membrane, which surrounds the cytoplasm. The cytoplasm contains the vacuole, the food storage granules, and the nucleus

Mushrooms and toadstools are fungi, but these are made of lots of cells, so they are not microbes. Yeasts are single-celled fungi, so they are microbes. Fungi are usually the biggest type of microbe. If there is just one of them, we call it a fungus.


A bacteria cell has a cell wall, which surrounds the cell membrane. Inside this is the cytoplasm. Bacteria cells have a chromosome instead of a nucleus

Bacteria are usually smaller than fungi. If there is just one of them, we call it a bacterium. Bacteria have many different shapes. Some have 'tails' (called flagella) that let them swim.


Diagram of a virus cell showing the protein coat around the edge of the cell and the strand of genesi instead of a nucleus.

Viruses are the smallest type of microbe. As a virus can only reproduce inside a cell, some people are not convinced that viruses are really living things.

Differences between fungi, bacteria and viruses

The table shows some of the similarities and differences between the three types of microbe.

Feature Fungi Bacteria Viruses
Cell membrane A green tick A green tick a red cross
Cell wall
A green tick
A green tick
a red cross
(protein coat)
Cell nucleus A green tick
a red cross
a red cross

Microbes - useful or not?

People often use the word germ instead of microbe, so you might think that microbes are all harmful. But some are useful to us.

Useful microbes

Yeast cells are useful to bakers and brewers. Yeast cells can change sugar into carbon dioxide gas and alcohol. This is useful to bakers because the gas helps the bread rise, and it is useful to brewers because it adds the alcohol needed for their drinks.

Bacteria are also useful to us. For example, certain bacteria cause the changes needed in milk to make yogurt and cheese out of it.

Harmful microbes

Many microbes can cause diseases. For example here are some diseases caused by fungi:

  • athlete's foot

  • thrush

Here are some diseases caused by bacteria:

  • tuberculosis, TB (affects the lungs)

  • salmonella (causes food poisoning)

  • whooping cough (affects the lungs)

Here are some diseases caused by viruses:

  • chicken pox (affects skin and nerves)

  • common cold

  • influenza, flu

  • measles (affects skin and lungs)

  • mumps (affects salivary glands)

  • rubella, german measles

Microbes cause disease when they are able to reproduce in the body. They produce harmful substances called toxins, and damage tissues and organs. We say that someone who has harmful disease-causing microbes in them is infected.

Spreading microbes

Many harmful microbes can pass from one person to another. Diseases caused by such microbes are said to be infectious diseases. Here are some ways that harmful microbes can be spread:

  • in air

  • through contact with animals

  • through contaminated food

  • through touch

  • in water


Droplets containing microbes fly into the air when people sneeze or cough. The microbes they contain get into other people if breathed in.

Chicken pox, colds, flu, measles and tuberculosis are spread like this.


Animals may carry harmful microbes. The microbes can get into a person who is scratched or bitten by such an animal. Malaria is a tropical disease spread by a tiny fly called a mosquito.


Food can have harmful microbes in and on it. The microbes get into the body when the food is eaten, causing food poisoning. Thorough cooking kills most microbes, but they can survive under-cooking. Careless handling of food increases the risk from harmful microbes.


Microbes can be passed from one person to another when people touch each other, or when they touch something an infected person has handled. Athlete's foot is spread like this.

Bacteria on the skin can be killed by antiseptics, and bacteria on surfaces can be killed by disinfectants. Washing your hands reduces the chance of spreading microbes.


Water can have harmful microbes in it. The microbes get into the body when the water is swallowed. Cholera is a disease caused by a bacterium that spreads like this. Thorough boiling or adding chlorine to the water can reduce the chance of spreading microbes in this way.

Defence against microbes

Natural barriers

The body has natural barriers to stop harmful microbes getting inside the body. Here are some of them:

  • acid in the stomach kills many microbes

  • sticky mucus in the lungs traps microbes, and then cilia sweep it out of the lungs

  • the skin stops microbes from getting into the body

  • scabs form on the skin if you get a cut, stopping microbes from getting into your body

  • tears contain substances that kill bacteria

The immune system

White blood cells

White blood cells

The body has an immune system that kills microbes if they get past the natural barriers. White blood cells are very important in the immune system. There are different sorts of these cells, but they can do two main jobs.

  • Some white blood cells can engulf microbes and kill them.

  • Some white blood cells can make substances called antibodies that stick to microbes.

Microbes have chemicals called antigens. Different microbes have different antigens. White blood cells have chemicals called antibodies.

White blood cells can stick to microbes if they have the right antibody to match the antigen on the microbe. When this happens the microbes can be killed, or clumped together to make it easier for other white blood cells to kill them.

Antibiotics and immunisation


Antibiotics are medicines used by doctors when harmful microbes have made you ill. They are substances that harm bacteria. Some antibiotics stop the bacteria reproducing and others kill the bacteria directly.

Antibiotics are helpful to treat diseases caused by bacteria, such as tuberculosis and food poisoning. They do not harm viruses, so antibiotics cannot treat diseases such as colds and flu, which are caused by viruses.

Antibiotics only work against bacteria, not viruses



When you are infected by a microbe, it takes time for your body to start fighting the infection. It does this by making enough white blood cells with the correct antibody. During this time, you continue to feel unwell.

You begin to recover when enough antibodies have been produced. After the microbes have been killed, the amount of antibodies goes down again. But some of the white blood cells that produce the correct antibody remain in your blood.

After a second infection by the same microbe, your body makes the correct antibodies much faster, because of the white blood cells that remain from when you had the first infection. The microbe doesn't get a chance to make you ill this time, and we say that you are immune to the microbe and the disease it causes.

Check your understanding of this by studying the animation.

The number of antibodies starts to increase at around day 2, reaching its peak at day 10. After this, the number of antibodies starts to decrease. At around day 17 the decrease starts to slow.


Immunisation is a process that doctors use to make people immune from certain illnesses, even before they have been infected. It involves you receiving an injection containing a vaccine.

Vaccines contain a dead or weak form of the disease-causing microbe, or some of its antigens. In response to the vaccine your immune system produces white blood cells with the correct antibody to kill the microbe, so you become immune without falling ill.

You are likely to have been immunised against several microbes, viruses and bacteria, including the ones that cause diphtheria, whooping cough, polio, tetanus, meningitis, measles, mumps and tuberculosis. Girls are also immunised against rubella.

Vaccination works against diseases caused by both bacteria and viruses.

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