The main source of viruses is what is known as Embedded HTML code, which is incorporated in the source code of an apparently normal web page. The user's machine is infected with the virus merely by visiting web pages which have either been designed to include or infected by these HTML macros. It is therefore essential to remain very vigilant with regard to the files you are sent. The best solution is to follow some basic security rules. The first is never to open an attachment which comes from an address that you don't recognize. However, contaminated files can easily come from people that you do know. It is therefore best to use an anti-virus program that will warn you of the presence of such malicious files. For more information, you can visit ENISA, The European Network and Information Security Agency.