There are only about 30 known viruses for the Macintosh, compared with thousands for DOS and Windows. No new Mac virus had been discovered for several years.

The virus is a 'worm', a self-contained and self-propagating program rather than one which attaches itself to another program.

The 9805 Autostart worm initially was spread through CDs using QuickTime's auto-play function.

Most reports claim the virus originated in Hong Kong, but some say it came from the mainland.

Mac repair shop Uptown Solutions' technical director Jim Ting says the virus began to spread about the same time as the OS 8.1 upgrade.

Mr Ting said the worm's author had been hard at work since then.

'Actually we have found three versions. The first two versions make your computer run slower,' he said, blaming the disappointing performance from the latest version of the Mac OS on the introduction of the worm.

The virus attempts to infect any hard disk, floppy or removable drive the computer comes into contact with, including volumes mounted over office computer networks, Mr Ting says.

The worm may not damage data but the amount of CPU time the program takes when looking for volumes to infect can bring the machine to a grinding halt.

Initial reports described the worm as PowerPC-specific and only transmitted through QuickTime's autostart function. However, several non-PPC servers at the South China Morning Post have been infected. None had QuickTime installed.

The latest variant, autostart worm 9508 C, is more vicious. 'The newly found version may damage some graphics files,' Mr Ting said.

The worm specifically looks for TIFF and EPS file formats, overwriting portions of the file with random data.

Virex can provide definitions to remove the latest versions. Uptown Solutions' stand-alone application Eradicator also will do the job.

hkmug.org.hk). Wormfood 1.2.1 also will handle all three variants.