Click fraud has always been an issue for advertisers using paid search, but for many, it has been considered a cost of doing business.

So long as campaigns are profitable, worrying about click fraud hasn’t always seemed like a priority.

But according to Click Forensics, the scourge of click fraud continues to grow and in Q4 2008, its Click Fraud Index, which tracks paid search campaigns across a network of search engines and advertisers, recorded its highest overall click fraud rate ever: 17.1%.

The click fraud rate for Google AdSense and the Yahoo Publisher Network was 28.2%, up from Q3 2008 but down slightly from the same quarter a year ago.

The most disturbing statistic: botnets accounted for 31.4% of fraudulent clicks, an increase of nearly 10% from Q4 2007. Because click fraud originating from botnets is often much more difficult (and sometimes impossible) to detect, their use is a troubling statistic. Google and Yahoo will definitely need to step up their game when it comes to fighting this type of fraud since it has the potential to wreak havoc if it continues to grow out of control.

But a tough economy brings out less sophisticated scammers too and some are using old techniques to cheat advertisers. Click Forensics’ president, Tom Cuthbert, stated that “we’ve started to see old schemes like click farms reemerge” and advised advertisers to be on the lookout for these types of threats.

Of course, the continued march of click fraud is not surprising but in these tough economic times when every dollar counts, search advertisers will need to be even more vigilant and should watch their campaigns like a hawk.