This article is from the Viruses and the Mac FAQ, by David Harley D.Harley@icrf.icnet.uk with numerous contributions by others.
There are around 40 Mac-specific viruses and related threats.
Mac users with Word 6 or versions of Word/Excel supporting Visual Basic
for Applications, however, are vulnerable to infection by macro
viruses which are specific to these applications. Indeed, these
viruses can, potentially, infect other files on any hardware
platform supporting these versions of these applications. I don't
know of a macro virus with a Mac-specific payload that actually
works at present, but such a payload is entirely possible.
Office 98 applications are in principle vulnerable to most of the
threats to which Office 97 applications are vulnerable. I'll return
to this subject when and if time allows. [DH]
Word Mac version 5.1 and below do not support WordBasic, and are
not, therefore, vulnerable to direct infection. Not only do these
versions not only understand embedded macros, but they can't read
the Word 6 file format unaided. There is, however, at least one
freeware utility which allows Word 5.x users to read Word 6 files.
This will not support execution of Word 6 (or WinWord 2) macros in
Word 5.x, so I would not expect either an infection routine or a
payload routine to be able to execute within this application.
However, Word 5.x users may contribute indirectly to the spread of
infected files across platforms and systems, since it is perfectly
possible for a user whose own system is uninfectable to act as a
conduit for the transmission of infected documents, whether or not
s/he reads it personally.
Files infected with a PC-specific file virus (this excludes macro
viruses) can only execute on a Macintosh running DOS or DOS/Windows
emulation, if then. They can, of course, spread across platforms
simply by copying infected files from one system to another.
DOS diskettes infected with a boot sector virus can be read on a
Mac with Apple File Exchange, PC Exchange, DOS Mounter etc. without
(normally) risk to the Mac. However, leaving such an infected disk
in the drive while booting an emulator such as SoftPC can mean that
the virus attempts to infect the logical PC drive with
I am aware of at least one instance of a Mac diskette which, when
read on a PC running a utility for reading Mac-formatted disks
after being infected with a boot-sector infector, became unreadable
as a consequence of the boot track infection.
Some Mac viruses may damage files on Sun systems running MAE or