Tony Fox, a spokesman for comedycentral.com, confirmed that the site hosted the virus until technicians discovered and removed the file on Saturday afternoon. Fox said the company is still trying to determine when and how the virus was placed on the server.
Joe Stewart, a researcher at Chicago-based Internet security firm Lurhq, said when the attacks first began Friday evening he spotted some ads linking to the same virus planted on the Web site of Lion's Gate Films. A spokeswoman declined to comment.
Santa Clara, Calif.-based anti-virus company McAfee Inc., detected more than 30,000 infection attempts since the attack started early Saturday morning, said Vincent Gullotto, senior director for the company's anti-virus emergency response team.
The virus tried to attack about 20,000 computers in the United States that use McAfee's anti-virus software for home users, Gullotto said. Another 9,000 attacks targeted European home users.
Toronto-based company Rydium, which places banner ads on Web sites using Falk's advertising technology, received a flood of calls Monday from customers who had gotten complaints that their Web sites were infecting visitors' computers, said spokeswoman Julie Ford.
The attack was similar to one last June that prompted warnings from U.S. government cyber-security officials. That virus infected the Web sites of the Kelley Blue Book automobile pricing guide and MinervaHealth Inc., a Jackson, Wyo., company that provides online financial services for hospitals and health care businesses.
The attack shows that the mere act of browsing the Internet has become a risky activity for many Internet users, said Marcus Sachs, a former White House cyber-security adviser and director of the SANS Internet Storm Center.
"It used to be that if you were a stupid or careless Web surfer, the worst that might happen is you get pop-ups and spyware installed on your PC," Sachs said. "But those rules have changed over the last year so that just by visiting a site -- even one you trust -- can bring not just a nuisance but serious damage."