IDG News Service — The Conficker worm is still infecting systems at a brisk rate and continues to snag computers in Fortune 1000 companies, according to security researchers.

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The worm is infecting about 50,000 new PCs each day, according to researchers at Symantec, who reported Wednesday that the U.S., Brazil and India have been hit the hardest.. "Much of the media hype seems to have died down around Conficker/Downadup, but it is still out there spreading far and wide," Symantec said in a blog post.

Conficker began spreading late last year, taking advantage of a recently patched flaw in Microsoft's Windows operating system to infect entire networks and also using removable storage devices to hop from PC to PC. Security experts say it has now infected millions of computers worldwide, which now comprise the world's biggest botnet network.

"We can see that companies that spend literally millions of dollars on equipment and gear to prevent infections … these Fortune companies have had this infection and it's stayed in their networks for a long period of time," said Rick Wesson, CEO of Support Intelligence and a member of the Conficker Working Group. "It's really hard and really expensive, and if the Fortune companies can't stop it, how can you expect small businesses to do it?"

The Working Group has set up so-called sinkhole servers that can communicate with infected machines. It has spotted infections within many Fortune 1000 companies, Wesson said. "Everybody got hit," he said. "Even Microsoft still has infections."

The worm got a lot of media attention in late March, and while the news stories have tapered off, the worm isn't going anywhere.

Some worried that an April 1 change in the way Conficker received updates could mark the beginning of a new round of Internet attacks, but in reality the Conficker network has been only lightly used, security experts say.

"It's still a significant botnet. It hasn't done anything of significance, but it has not gone away," said Andre DiMino, cofounder of The Shadowserver Foundation and a member of the Working Group. "The remediations need to ramp up."

"This thing is not dead," he added. "Everyone has kind of passed it over, but it's not dead."