Google has begun issuing warnings to millions of people that their PC has been infected with a virus.
The malicious code pipes browser traffic through sites that promote the scammers' wares which include fake security programs.
Those hit by the virus will be warned with a message that will appear at the top of searches carried out via Google.
The search firm estimates that more than two million people have been hit by the infection.
Google uncovered the huge number of infected machines while doing routine maintenance on a data centre.
During maintenance, Google servers get taken offline and typically this means search traffic for that cluster of machines dries up.
However, wrote Damian Menscher on the official Google blog, switching off one cluster did not stop all traffic.
Investigation revealed that the traffic was being generated by a virus on perhaps a million Windows machines. The virus bounced packets of data off the net address of the Google servers to find out if they were online.
"The malware appears to have gotten onto users' computers from one of roughly a hundred variants of fake anti-virus, or 'fake AV' software that has been in circulation for a while," wrote Mr Menscher.
The main effect of the virus seems to be to funnel search requests through intermediate sites that promote fake security programs and other scams.
Google will be putting a warning at the top of search results seen by people with a machine known to have bounced data off the Google servers. The warning contains a link to advice pages that help people update their anti-virus and clean up their PC.
So far, said Google, it has warned "hundreds of thousands" of users and expects to notify many more.