Google is warning users who appear to be infected with a widespread malware infection when they use Google’s search product, the company said today.

Google engineers noticed suspicious search data recently and worked with IT security at affected companies to find the cause: malware that re-routes traffic to Google and other sites through a proxy. To what end the infection does this for is unknown.

Google does not have the ability to detect the presence of viruses on user’s computers, simply the suspicious network activity caused by the malware.

Google says it hopes to combat the spread of the infection by making sure users are aware of it and can update their anti-virus software and catch it. If you are infected, you’ll know — the company is using a big, yellow warning that you can’t miss.

MalwareWarningScreenshot Google warns users of widespread Windows only malware infection

Google’s Matt Cutts says:

This is important: Google has been able to detect a large number of computers infected with a specific piece of malware. If you go to Google and do a search (any word will do) right now, check to see whether you get a “Your computer appears to be infected” warning at the top of the search results. If you see the message, you need to clean up the infection from your machine.

He also notes that the infection only affects machines running Windows.