Paul Ehrlich, gifted with knowledge, vision, and the capacity to bring his discoveries to practice, shaped the destiny of many biomedical scientific disciplines, including immunology, chemotherapy, hematology, cytology, and cancer research. His perceptive concept of receptors and ligands binding together in highly specific reactions was introduced just over a century ago in 1898 and is both fundamental and central to present day biomedical research. His mother country, Germany, commemorates his seminal contributions to science and to human welfare with an annual prize for achievements in fields that are related to his work. The 1999 Paul Ehrlich and Ludwig Darmsteaedter Prize was awarded to Robert C. Gallo, M.D. for his achievements in the pursuit of cancer related viruses and the growth in culture of human T-cells which led to the discovery of the first human retroviruses and, as a direct consequence, the discovery of the Human Immunodeficiency Virus, the third known human retrovirus. Below, excerpted from his acceptance lecture, is a concise personal history of those discoveries.