http://www.dmturner.org/Teacher/Library/5thText/SimplePart4.html
Viruses
Is a cold caused by bacteria? No, the cause of a cold is a virus. A
virus has DNA or similar materials in it, as cells do, and it is made
out of the proteins which are the building blocks of cells, but it is
not a cell. The virus does not belong to a kingdom of living things,
because it is not clearly alive. It is a good example of the
messiness of the natural world. Things do not always fit our
explanations, which is why scientists keep looking for better
explanations. 
Virus reproduction
Viruses, obviously, cannot make seeds or spores or eggs. They are not
cells, so they cannot divide like protists and monerans. Yet they
reproduce themselves. They do this in a fascinating way. 
The virus attaches itself to a cell. It invades the cell and puts its
DNA into the cell. DNA contains instructions which the cell must
follow. The instructions are, very simply, to make more viruses just
like the invader. The cell begins making viruses, and does not stop.
Sometimes the new viruses fill the cell and make it explode. Other
times the virus stays inside the cell for some time. Either way, a
virus can make the organism it infects very ill. 

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