PCWorld: Security researchers from vendor ESET have issued warnings that the TDL4 rootkit is getting better at avoiding detection by antivirus software. The newest version creates a hidden partition at the end of a hard drive and sets it to active. That makes it more difficult for antivirus software to find the malware.
Even before this new variant emerged, experts from Kaspersky Lab called TDL4 "the most sophisticated threat in the world." They estimate that at least 4.5 million systems are infected with the rootkit.