I once gave blood just after coming back from the Alps and having a cold sore. They took my blood after checking with the doctor in charge. The next time after 6 months they were specifically requesting that they wanted blood with antibodies to Herpes simplex, I said that I regularly used to get cold sores after being exposed to light and that I had recently had quite a bad cold sore. They looked at their records and found that 6 months earlier I had given blood with a cold sore, they said thank you very much and took my blood again. Here in France they won't let me give blood as they think I might be contaminated with CJD.