Malware infections have more than doubled since April 2009. Search results containing a url labeled as harmful have remained level in the range of 0.5% to 0.9%, an improvement. While the web as a whole has become more dangerous, Google’s been doing an even better job clearing their search results.
http://searchengineland.com/how-to-avoid-getting-your-search-rankings-trashed-by-malware-25199