Macro viruses are the most commonly occurring at present. They spread when a contaminated Microsoft Word, Excel or PowerPoint document is opened. Originally macro programming language was intended to be used to create shortcuts for repetitive tasks, for instance to allow a user to print, save and close a document with a single keystroke. Macro viruses can spread rapidly. When one opens a document to which the virus has attached itself, it infects that document and then goes on to infect all documents subsequently opened by the application. As Word, Excel and PowerPoint documents are frequently shared over the internet, this creates a vector for the rapid spread of these viruses. Moreover, macro programming language is much easier to learn and less complicated than traditional programming languages. Although these viruses are very common, they are also very easy to tackle with an up-to-date anti-virus program. For more information, you can visit ENISA, The European Network and Information Security Agency.