On reading the first sentence of the preface to the Cecil Textbook of Medicine we are warned that "Medicine is forever mutable." This should not surprise us. New diagnostic techniques and therapeutic agents frequently appear before we get to know the old ones, and our understanding of the cellular basis of disease undergoes similar continual revision.
These man-made discoveries, however, are only part of the torrent of new scientific information. Sometimes changes in the biosphere result in devastating effects on medicine. An example is the acquired immunodeficiency syndrome: AIDS was not mentioned at all in the 1982 (16th) edition of this text and first appeared as an important but brief entry in the 1985 (17th) edition. It occupies a major space allocation in the present edition. Because of this glut of new facts and concepts, the proper frequency of textbook revisions becomes a critical matter for both authors and readers.