Viruses, plagues, and history. A general introduction. Introduction to the principles of virology. Introduction to the principles of immunology -- Success stories. Smallpox. Yellow fever. Measles virus. Poliomyelitis -- Present and future challenges. An overview of newly emerging viral plagues : the hemorrhagic fevers. Lassa fever. Ebola. Hantavirus. Severe acute respiratory syndrome (SARS) : the first pandemic of the twenty-first century. West Nile virus : deaths of crows and humans. Human immunodeficiency virus (HIV) : AIDS, the current plague. Mad cow disease and Englishmen : spongiform encephalopathies-Prion disease. Influenza virus, the plague that may return. Conclusions and future predictions.