The BBC
reports that an analysis by Google of some 4.5 million web pages found that
10% of them contained malicious code.
These pages contain what are known as "drive-by" infections, malware that is downloaded to your computer in the background while you're looking at what's on-screen. From the article,
"The user is presented with links that promise access to 'interesting' pages with explicit pornographic content, copyrighted software or media. A common example are sites that display thumbnails to adult videos.
"The vast majority exploit vulnerabilities in Microsoft's Internet Explorer browser to install themselves.
"Some downloads, such as those that alter bookmarks, install unwanted toolbars or change the start page of a browser, are an annoyance. But increasingly, criminals are using drive-bys to install keyloggers that steal login and password information. "
The scale of the problem boggles the mind.
Edit: The complete paper is available
here.