When accepting blood, how do you know that there are no disease or viruses in that donated blood?

or:When accepting blood, how do you know that there are no disease or viruses in that donated blood?or:They test it before hand. Now, it's always po

or:When accepting blood, how do you know that there are no disease or viruses in that donated blood?

or:They test it before hand. Now, it's always possible there could be, but it must be incredibly rare because I've personally never read of an incidence where there was.

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