Well, technically Downloader.GK is a Trojan, not a virus. A Trojan does not spread on its own (unlike a virus), but its creator usually trick the user to launch it by lying on its purpose.

Downloader.GK infects users going to specific websites designed to spread it. The website asks the user to let it install an "Active X" application and subsequently installs the Trojan.

Panda Software has ranked the Trojan the "Top virus" because it made 14% of all virus attacks this year. It runs spyware and adware programs on infected computers.

Panda Software Downloader.GK information

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