Microsoft has just released the latest edition of the Microsoft Security Intelligence Report, and one bit of information gleaned from it – that comes as no surprise – is that the method most frequently used by malware distributors to propagate malicious code is socially engineered malware (i.e., malware that requires user interaction to execute). In fact, 44% of the malware detected (in a sampling of approximately 600 million computers all over the world) fell into that category. To find out more and see what other methods are being used, see Which is the most popular malware propagation tactic? on ZDNet:
http://www.zdnet.com/blog/security/which-is-the-most-popular-malware-propagation-tactic/9638