[Alex]

I understand barring EPO, but not simple transfusions. That article lists as the negative of transfusions:

Both types of transfusion can be dangerous because of the risk of infection and the potential toxicity of improperly stored blood. Homologous transfusions present the additional risks of communication of infectious diseases and the possibility of a transfusion reaction. From a logistical standpoint, either type of transfusion requires the athlete to surreptitiously transport frozen RBCs, thaw and re-infuse them in a non-clinical setting and then dispose of the medical paraphernalia.

This is only true because it is banned. If it weren't banned there'd really be no risk of infection and toxicity because they wouldn't have to try and hide it. There is the same risk with improperly stored salmon but they're still allowed to eat that if they want.