There are many reasons for the modern interest in viral vaccines, but there is no doubt that the key role played by viral vaccines in public health is the major factor since other prophylactic or therapeutic anti-viral products simply do not exist. Viral vaccines have a long history that has been marked by successful events and by tragic accidents. Live viral vaccines are an extraordinary category of biologicals since, despite their reputed efficacy, they were developed by empirical experiments and patient epidemiological observation. From this point of view oral polio vaccine should be considered a 'miracle' since it became a major tool for public health in the 20th century, before we were able to understand the molecular basis of polio virus neurovirulence attenuation. The first evidence that polio virus can be attenuated was provided in the early 1940s by Max Theiler, but it was Hilary Koprowsky who demonstrated further in 1952, that a rodent adapted strain was safe and able to immunise a limited number of volunteers. Koprowsky studies were confirmed later during a mass field trial in Africa. However it is undeniable that the patient and systematic work of Albert B. Sabin was primordial in developing live oral attenuated poliovaccine. The excellence of Sabin's testing of poliovirus neurovirulence in the accurate studies that he developed, enabled him to select, after the cloning of viral populations by plaque assay, the best attenuated variants.(ABSTRACT TRUNCATED AT 250 WORDS)