Now Transplants Tainted Too?!

At least it looks like this one hasn’t got anything to do with China for a change.

FEDS PROBE TAINTED TRANSPLANTS

Federal officials are investigating what three hospitals knew and told four organ
transplant patients about a high-risk donor who infected them with HIV and hepatitis.

Now I am not a hamster, I thought that transplant donor materials were screened for things like HIV and hepatitis? If that’s so, how does something like this manage to happen? and what DO those hospitals know about the situation and what could they have prevented?