22. Subjects will be excluded from the study if they are considered by the investigator to be at risk of transmitting through blood and other body fluids the agents responsible for AIDS (Acquired Immunodeficiency Syndrome) or other sexually transmitted disease or hepatitis. This will be achieved by the use of a card (similar to that used by the National Blood Transfusion Service) which asks a potential subject if they have reason to believe that they may fall into any category included on the card. If the verbal answer is in the affirmative then they will be excluded from the study