While they've tried to mitigate the risks, only your own blood is truly safe for transfusion. From hepatitis to Hiv, all can be transfered in a transfusion. If possible, have enough of your blood drawn to be used if surgery, or another medical procedure is needed soon for you.
They used to be, but now they know bloodtypes and all that stuff, so there's really no risk. I don't think that they allow people with STD's like AIDS to be donors of blood. No offense if I offended anyone somehow.
If you have an allergic reaction without treatment you will die of shock. Thats why the nurse giving blood must stay with the pt for the first 15 min of the transfusion and start at a slow rate. Vital signs and assessment are done before, during and post transfusion for at least 6 hrs. Reaction is treatable though if recognized early.