I just read up on this and the list of people they don't want blood from is pretty vast. I wouldn't be allowed to give blood myself because I've just had a tattoo done in the last 4 months.
Here's a list from the NHS. I suppose they're just trying to be careful.
Quote:You may not be able to donate blood if:
you have had a serious illness or major surgery in the past
you have had complicated dental work (it is safe to donate blood 24 hours after having a filling or seven days after a simple extraction)
you have recently come into contact with an infectious disease
you have had certain immunisations within the last four weeks
you are currently on a hospital waiting list, or waiting to have tests
You should not give blood if:
you have a chesty cough, sore throat or an active cold sore
you are taking antibiotics or have finished a course of antibiotics in the last seven days
you are pregnant or have given birth in the last six months
you have had hepatitis A or jaundice in the last 12 months
you have had an ear piercing or body piercing in the last four months
you have had a tattoo in the last four months
a member of your immediate family has had Creutzfeldt-Jakob disease (CJD), a rare condition that affects the nervous system and causes brain damage
you have had acupuncture in the last four months, unless this was done within the NHS or by a qualified healthcare professional registered with a statutory body
you have visited a part of the world where malaria is common in the last six months
you have received human pituitary extract (a substance used in some growth hormone and fertility treatments before 1985)
you have received blood during the course of a medical treatment or procedure since 1980
You should not donate blood for 12 months after having sex with:
a prostitute
someone who has injected drugs
someone who has haemophilia (a condition that stops your blood from clotting normally)
someone who has been sexually active in parts of the world where AIDS and HIV are common, such as sub-Saharan Africa
a man who has had oral or anal sex with another man (if you are female)
a man (if you are male)
As of November 2011, a man who has sex with men can donate blood provided they have not had anal or oral sex with a man during the past 12 months (even with a condom). Previously, men who had sex with men were not allowed to donate blood.
You should never donate blood if:
you have ever had HIV
you have ever had hepatitis C
you have ever had syphilis
you have ever had human t-lymphotropic virus (HTLV)
you have ever injected yourself with drugs
you have ever worked as a prostitute
Here's a list from the NHS. I suppose they're just trying to be careful.
Are you ready for the fire? We are firemen. WE ARE FIREMEN! The heat doesn’t bother us. We live in the heat. We train in the heat. It tells us that we’re ready, we’re at home, we’re where we’re supposed to be. Flames don’t intimidate us. What do we do? We control the flame. We control them. We move the flames where we want to. And then we extinguish them.
Impersonation is treason.