Just how big of an issue is malware on the web? With the number of websites I visit every day,. I was surprise to learn that on average, it seems more than 10% of pages in a 4.5 million sample were found to have some form of malicious code in them. A Google research team found over 450,000 pages that tried to install trojans or spyware on vulnerable machines, exploiting unpatched vulnerabilities. Interestingly enough, it seems that the actual owner of the site was often not to blame. Rather, it was the advertisers that were lacing their content with bad code:

It found that the code was often contained in those parts of the website not designed or controlled by the website owner, such as banner adverts and widgets.
The ethics of online advertising aside, you'd think it would be counter-productive to get identified as a company that peddles malware. Then again, peddling adware is par for the course and apparently very profitable. Malware is just one step further. Given the sheer number of sites that depend on ad revenue to stay afloat, perhaps 10% isn't that surprising of a number. It's still sickening.