Click fraud is reaching epidemic proportions, according to a new report by Click Forensics, which said the nefarious practice has climbed to 15.8 percent of all ad clicks for the second quarter of 2007.

"Click fraud has become the new spam," said Tom Cuthbert, president and CEO of Click Forensics. "It's clearly getting worse, not better."

The report came from data compiled by more than 4,000 advertisers. Perpetrators of click fraud are increasingly turning to malware to infect PCs, gathering the unwitting computers in a massive botnet capable of inundating a site with useless clicks.

According to Click Forensics, industry leaders such as Yahoo! and Google have largely ignored the problem. The study found that Yahoo has a click fraud rate of 25.6 percent, compared to 21.9 percent for Google.

"When we say there's something going on, Google tells us to document it, Pepperjam CEO Kris Jones told Forbes. "We say we don't have enough information to document it, and it goes back and forth like that. So most advertisers just chalk it up to one of the costs of doing business."