The first Thorvald Madsen Memorial Lecture commemorates the 100th anniversary, 18 February 1970, of Madsen's birth. The nature of his work at the State Serum Institute, Copenhagen, and as President of the Health Section, League of Nations (now WHO), particularly in the control of infectious diseases, is memorialized by this overview of viral diseases that produce permanent or semipermanent immunity. Emphasis is placed on the use of attenuation of certain viruses to produce an image of immunity to the natural diseases by greatly modified infections with mitigation of the severe symptoms. Particular attention is given to the work of the author and his colleagues on attenuated virus vaccines for measles, mumps, and rubella, and on the use of a single injection of combined dual vaccines (measles and mumps, mumps and rubella) and of a combined triple vaccine (measles, mumps, and rubella). Denmark's contribution to the comity of nations and Thorvald Madsen's share in that contribution are stressed.