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A month after the first customers called its support lines for help, Apple responded to the Mac Defender outbreak with a security update that attempts to block new infections and remove malware that's already been installed. But the bad guys haven't been standing still. They've renamed their hostile software (Mac Shield) and produced at least 15 new versions, forcing Apple to respond with a new set of definitions every day this month.
For more details, see "Has Apple done enough to fight malware on Macs?"