What does email have to do with opening Word documents with macro viruses? What does email have to do with opening any data file that maliciously exploits the

What does email have to do with opening Word documents with macro viruses? What does email have to do with opening any data file that maliciously exploits the application opening the data file? How can a whitelist prevent attack vectors as varied as opening a PDF file designed to exploit your PDF reader? The user cannot always be vigilant enough to know that site X is giving him a malicious file to open. Hell, site X might actually be a legitimate site that has itself been attacked and exploited...