Reply #108: Pretty sure, yeah. They test the blood

[Ilsa]

in batches, you know, and most blood comes back negative, and if one comes back positive, they isolate the batch until the determine which unit is undesired.

In San Antonio a few years ago, a "bad" unit got through and infected several people since the blood had been split into components (plasma, RBC, etc). The level of infection was below the threshold of detection, according to the bloodbank. IIRC, they said the blood came from a donor who probably had a sexual encounter with a person they didn't know was bi or was using injected street drugs.