Back in the 1980's & 1990's, few things induced such a fear with
computer beginners than computer viruses. The media ran big bold
headlines about viruses infecting company networks and Macintosh
publications didn't really think different (pun intended).
Viruses & worms became a typical "fill-in topic" for magazines
and computer TV shows. 

In hindsight, many reports were exaggerating or dramatizing,
especially on the Macintosh side of things. Especially by
comparison to the ransomware we 'enjoy' on modern gear today. But
System 7 hasn't been invulnerable either. So there's a few
troublemakers around which may make inroads onto your System 7
Macs from downloads and old media. 


How do I know my System 7 installation is infected?

Sometimes file sizes seem to randomly increase for no reason. Or
your Extensions folder holds items you never recall installing.
Do your desktop icons suddenly change to the default hand &
pencil icon? Those could be indicators - but it's hard to
diagnose without the right software. 

System 7 Today recommends using Disinfectant for that. It scans
your HDD and cleans it of any found intruders. Disinfectant has a
great track record and seems to find all the most popular viruses
from the old days, including the dreaded nVIR one. 


What about viruses for pre System 7 Macs?

There's been a couple of viruses for pre System 7 Macs. Most of
them are harmless anyway and we don't list them here as they pose
no threat to your System 7 Mac. A lot befall the WDEF and CDEF
resources of your desktop file, but won't be active on System 7.
Disinfectant finds and exterminates them anyway. 


What follows is a short list of the more proliferated System 7
viruses for the Mac:

	ANTI
Infects only applications (including Finder) and tends to clear
CODE 1 resources. Files are not repairable as e.g. an anti-virus
program like Disinfectant has no means to know the original
values. ANTI broke with System 7.0 and does not replicate (it
does in System 6 & earlier!) 

	Autostart 
Very annoying worm that infects PowerPC Macs and spreads itself
even to mounted network drives. Uses the QuickTime AutoPlay
feature for infiltration, so one easy way to stop it is to
deactive CD-ROM AutoPlay in the Quicktime control panel. 

	INIT 29
Infects all files. If you insert a write-protected floppy disk an
alert window will pop up and tell you about necessary minor
repairs on the floppy. Apart from proliferation it is relatively
harmless.

	INIT 1984
Evil one. Activates itself every friday 13th and starts to
replace folder and file names with random characters. Also
randomly changes creator and file types of files Init-M is an
alteration of it, known to rename files as “Virus MindCrime”.

	HC
This virus infects only HyperCard stacks and can only spread
through HyperCard stacks, which makes this an extremely rare one.
Indication of infection: When running an infected stack, the Mac
may hum strangely and HyperCard painting tool symbols will appear
at random parts of the screen. 

	MODM
Rare occurence. Infects System file and application files. May
cause system crashes on infected computers. Also known as the
"zero virus". 

	nVIR
The most widely spread Mac OS virus. Infects system folder & apps
and adds a "nVIR" resource to them. When the System file is first
infected, a counter is initialized to 1000. The counter is
decremented by one each time the system is started up and it is
decremented by two each time an infected application is run. When
the counter reaches zero, nVIR will sometimes either say "Don't
panic" (if MacinTalk is installed in the System folder) or beep. 

	Scores
Rather widespread. Creates the invisible files "Scores" and
"Desktop" in your System Folder. Apart from proliferation, it
doesn't seem to do much harm. One symptom of a Scores infection
is lost icons for Note Pad and Scrapbook files. See example here.
Disinfectant kills it reliably.

	WDEF
Infects the invisible "Desktop" files used by the Finder.
Fortunately, System 7 seems immune to the virus. Rebuilding the
desktop usually gets rid of it. If you use ResEdit to search for
WDEF resources, do not be alarmed if you find them in files other
than the Finder Desktop files. WDEF resources are a normal part
of Mac OS. Any WDEF resource in a Finder Desktop file, however,
is a reason for concern. 

	ZUC
Aka the "drunken mouse cursor" virus. If your Mac is befallen,
the cursor will act like a drunk once you press the mouse button.
It'll stop once you release the mouse button. It is not known to
cause other trouble.
.