Giving blood exception - eviltoast

I was listening to the American military radio here in Korea and they invited soldiers to donate blood with some exceptions:

  • if you’ve been shorter than 30 days in Korea
  • if you got a tatoo in the last 6 months
  • if you lived in Europe for more than 5 years

I really wonder what could have happened to someone who lived in Europe for more than 5 years which triggered this particular exception.