Why can't you give blood when you barely got a piercing?

The blood bank has no way to be comfortable with taking the risk that your piercing hasn't left you with some sort of disease from dirty equipment or do-it-yourself piercings. They typically want to see if you are alive and healthy after 6-months before they take the risk.

I ate British beef while I was stationed in Spain in the early '80's and they still won't take my blood because of the risk of "Mad Cow" disease...moo.