An analyst looks at code in the malware lab of a cyber security defense lab at the Idaho National Laboratory in Idaho Falls, Idaho. REUTERS/Jim Urquhart Online ad fraud costs advertisers more than $7 million every month, as they pay for digital ads that were never actually seen by humans.

Much of this online ad fraud is created by botnets: armies of PCs infected with malware that generates thousands of fake clicks on ads. The botnet controllers tend to be unethical web publishers that want to ramp up the prices of advertising on their sites by inflating the amount of clicks on their sites.

Ad fraud has a number of drastic consequences. For users, it can significantly slow down their machine. For advertisers, it means they have to pay more to actually generate meaningful results from their online advertising. And for ethical publishers and advertising technology companies, it diminishes trust in the digital advertising industry, which could lead to less spending.

But while the consequences for all those involved are dire, it can be difficult to really understand how ad fraud works in practice.

That’s why fraud detection company Forensiq has created a video to highlight just how quickly a computer infected with malware can start racking up thousands of false ad impressions.

Computers usually get infected with this type of malware when a user clicks on a link or visits an unsafe or compromised website.

And off it goes. On the left, the video shows the hundreds of windows that are being opened, hidden by the user. (On the right is a list of processes occurring on your computer, such as cookies being dropped from the hidden sites.)

Big advertisers are affected by ad fraud: P&G below has just had to pay for the click on this particular ad for Tide on the right-hand-side of the web page, despite the fact that it was never seen by a human.

The bot works relentlessly as long as the machine is on. After 24 hours, 10,000 false impressions are created from just one machine. If there were 100,000 machines in a bot net, that could equal 1 billion fraudulent impressions a day.

Here's the full video: