Google yesterday said that 1 in 20 personal computers is infected with what it called an “ad injector” as it previewed a few factoids from research it will publish in just over a month.

“More than 5% of people visiting Google sites have at least one ad injector installed,” said Nav Jagpal, a software engineer with Google’s Safe Browsing team, in a blog post Tuesday. “Within that group, half have at least two injectors installed and nearly one-third have at least four installed.”

“Ad injectors” are just what they sound like: Small programs that insert online advertisements into a Web page, typically without authorization from either the device owner or the publisher of the website. Superfish Visual Discovery, the ad injector that blew up in Lenovo’s face last month, was a very prominent example.

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