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This is because these viruses are more powerful than the resisting mechanisms that is WBC's (white blood cells that protects our body from diseases) & manage to kill the wbc's.

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Its true that we have a resisting system but is also true that the viruses are also capable to destroy our immune system(WBCs ).The virus use up the metabolism of the host and they do make us weak ,they are the most versatile form of living organisms . HOPE THIS HELPED U BUDDY. NYC TO HELP U