Click fraud is to Google and Yahoo what bird flu is to world health -- a problem that's manageable today, but could grow into a crisis without aggressive prevention efforts.

Silicon Valley's twin titans of the Internet era get almost all their revenues from pay-per-click advertising.

Advertisers hire Google and Yahoo to distribute the online equivalent of billboards that take people to the advertisers' sites. But advertisers pay only when someone is interested enough to click on their links.

There are several variants of pay-per-click advertising, but one form is particularly relevant to click fraud: affiliate networks.

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