CHICAGO —  Government and industry experts warned late Thursday of a mysterious, large-scale Internet attack against thousands of popular Web sites. The virus-like infection tries to implant hacker software onto the computers of all Web site visitors.

Industry experts and the Homeland Security Department (search) were studying the infection to determine how it spreads across Web sites and find adequate defenses against it.

"Users should be aware that any Web site, even those that may be trusted by the user, may be affected by this activity and thus contain potentially malicious code," the government warned in one Internet alert.

The mysterious infection appeared to target at least one recent version of software by Microsoft Corp. (search) to operate Web sites, called its Internet Information Server, popular among businesses and organizations.

A spokesman for Microsoft declined to comment immediately.

Experts said the attack's effects were unusually broad but weren't substantially interfering with Internet traffic.

"While this is significant, it has no impact on the operation of the Internet," said Marcus Sachs, who helps run the industry's Internet Storm Center (search) in Bethesda, Md.

Experts urgently recommended consumers and corporate employees to update the antivirus software on their computers, since the latest versions can immunize visitors to infected Web sites.

The infected Web sites attempt to implant on visitors' computers hacker software that allows others to use their computers to surreptitiously route Internet spam e-mails.