Bacteria reproduce by making copies of themselves. To do this they need a source of nutrients for energy and they need warm, moist conditions so the chemical reactions inside them can take place.
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Unfortunately for us, there are lots of places inside the human body where you find these conditions, e.g. your intestines, so a few bacteria can quickly become a large colony inside the body.
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Viruses need other cells to reproduce - they use parts of the other cells to make copies of themselves.
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Certain viruses can easily reproduce inside the human body as there are lots of the right kind of cells for them to use.
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