via a recent emdocs article (click through for the full read), some ballpark numbers that 
might help you when you consent your next patient for a blood transfusion:
<1% (~0.63%) had a transfusion reaction, based on data from 2011
only 317/8,000,000 (~ 1 in 25,000) had a serious reaction requiring ICU-level care
risk of viral transmission:
- HIV -> 1: 1,467,000 units
- Hepatitis C -> 1: 1,149,000 units
- Hepatitis B -> 1: 357,000 units
in comparison, for reference:
- odds of a royal flush (5-card poker): ~1 in 650,000
- odds of a four-of-a-kind (5-card poker): ~1 in 4000
- odds (in a single year) of dying in a motor vehicle accident: ~1 in 9000
- odds (in a single year) of dying in an “air and space transport” accident: ~1 in 770,000
References: emdocs article; poker probabilities; mortality risk
