That's why it is very important to pre screen and not allow people with high risk behavior to add to the pool.
I've only donated blood once, because my blood pressure is too low. When I went, to Medic, I was given a generic form to fill out. I was honest...I had no reason not to be.
But if someone is getting paid ( I wasn't) for their blood...and need the money...are they really going to be totally honest and risk not getting any money?
Filling out a form, does not tell the blood bank that one's blood is contaminated. I know Medic tests
after, and will send out a letter to the donor if there are any problems. (I received one, my cholesterol was high) And I think by law, they are required to report to the health dept if AIDs, Hep C or any STD is present.
It can take years for HIV/AIDS symptoms to show up, yet it may be present in the blood. I think it can take some extended time for Hep C to show up too. A close friend ended up with Hep C from a blood transfusion during extensive abdominal surgery (he's a "bleeder", and is the father of the son who had hemophilia and died from AIDS from contaminated blood. Two of his daughter's 3 sons are also hemophiliacs. They can fall riding a bike and require a transfusion of factor (packed cells)).
There are many many people who end up with AIDS and Hep C through no fault of their own.....all because of a contaminated blood transfusion or being a medical care person who contracts either disease through their care of an infected patient.