Viruses are often classified according to the objects they infect. Macro viruses, as the name suggests, are designed to add their code to the macros associated with documents, spreadsheets and other data files. The vast majority of macro viruses were designed to spread on the back of Microsoft Office data files (Word, Excel, Access, PowerPoint and Project), although there were a few ‘proof-of-concept’ macro viruses for other formats (Lotus AmiPro, for example).