Computer viruses pose a considerable problem for users of personal computers. The recent emergence of macro viruses as a problem of some importance may heighten virus awareness in general. Yet most people have little or no understanding of common anti-virus measures, the varieties of viruses that exist today, and the strategies which they use to accomplish infection and to defeat anti-viruses. It is well-known that the virus problem is most severe for users of IBM PCs and compatibles; however, users of other platforms, such as the Macintosh, should not become complacent - viruses exist for many platforms in varying numbers. The ease with which macro viruses may be written is discussed, and a new virus attack for the Macintosh is presented which closely resembles an attack under DOS for the PC.