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The Invaders
We often say that 'germs' cause diseases. What do we mean by this?
We mean that microbes invade our bodies and cause damage. The commonest types of microbes
are bacteria and viruses.
Other microbes that rarely cause damage include some microscopic parasites and fungi.
Bacteria are fairly small cells and have a variey of different shapes.
These are:
Speheres known as 'cocci'.
Rods know as 'bacilli'.
Spirals know as 'spirilli'.
Unlike other cells, bacterial cells have no nucleus.Their genetic material (DNA) is free within
the cytoplasm. They replicate themselves by dividing into two.
A cell wall surrounds bacterial cells but is not made of cellulose like plant cell walls. This cell wall
gives protection to the bacterial cell membrane and shape to the cell.
Some bacteria have a small tail called a flagellum which is thrashed about to propel them. Others
have multiple smaller versions of this called cilia.
Harmful bacteria make us ill by either damaging our cells or producing poisonous toxins.
But we are full of nice, friendly bacteria too which help to keep us healthy, for example in our digestive
They are not cells, they are much tinier and cannot replicate themselves. Like bacteria, viruses come
in all sorts of different shapes.
No matter what shape they are viruses share some common features. They have no
nucleus. Instead they have a surrounding protein coat that gives them their unique shape. Inside
this is a string of DNA.
The only way viruses can make us ill is to get themselves into our cells.
So viruses can get into our cells in a variety of ways including landing on our cells and injecting their
DNA into them. Others break down the cell membranes then sneak inside. Once inside our cells they
hijack them and make millions of copies of themselves. Each of these can go off and invade other

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How does the body fight back?
Passive Protection
Passive defences are those that are set up to stop bacteria or viruses entering the body. They act as
roadblocks. They are found in those places where the invaders are most likely to try to the body.
Invaders try to get inside us via five main areas:
1. The Skin
2. The Eye
3. The Respiratory System
4. The Reproductive System
5. The Digestive System
Microbes also use another means of access.…read more


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