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Viruses are even smaller than bacteria. They cause more illness than any other kind of disease germ.
Viruses harm your body by entering and damaging body cells. Inside the cell the outer coat of the virus breaks down, and the virus reproduces rapidly. The new viruses use the cells energy and crowd the inside of the cell, keeping it from working properly. The cell dies when the viruses breaks out of it. Then the virus invades other cells. As with other kinds of disease germs, a small number of viruses are not harmful. If the viruses reproduce, however, they can damage so many cells that you can become sick. Personal use only
In the last fifty years,scientist have identified hundreds of different kinds of viruses. Each kind of virus causes only one disease.