A new computer trojan has infected 550,000 machines running Apple's Mac OS X - and many could still be vulnerable.
The infected machines are now part of a 'botnet' of zombie machines which can be controlled by cyber criminals and 'told' to download new malicious software.
The attack has been described as a 'rude awakening' for Mac users.
The attack has afflicted machines in America and the UK worst, according to Russian security vendor Dr Web's statistics
The new attack was spotted by Russian anti-virus vendor Dr Web.
'We conducted research to determine the scale of spreading of Trojan BackDoor.Flashback that infects computers running Mac OS X,' says the Russian antivirus vendor.
'The botnet encompasses more than 550 000 infected machines, most of which are located in the United States and Canada.
'This once again refutes claims by some experts that there are no cyber-threats to Mac OS X.'
Many Mac users believe that the system is 'immune' to viruses and trojans - but in reality hackers rarely choose to write viruses that afflict Macs, simply because there are relatively few Macs compared to the huge number of machines that run Windows.
Hackers want their attacks to spread rapidly, so they usually target the OS that offers the most potential 'victims'.
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