Even though the discovery of blood groups and the proposal of effective anticoagulants made transfusion between different individuals compatible, blood recipients had to face another relevant problem: the transmission of viruses carried in the blood and its fractions. Some of these viruses, e.g., hepatitis B and C, cause important liver complications, but another, the HIV (Human Immunodeficiency Virus) has caused an epidemic that had relevant health complications and social consequences. Only with the identification of these viruses, the development of methods capable of eliminating them and the setting up of effective screening did transfusion become very safe and its use widespread.