This is a whole topic unto itself, and largely out-of-scope
for these pages. However, there is apparantly one
binary virus for Linux (bliss).
Loosely speaking, viruses may be cast in one or more categories:
Boot sector viruses - spread by floppy disk
Binary viruses - spread by downloading programs, infecting other
programs.
Macro viruses - spread by email and documents, infecting Word
etc. templates and documents.
Info viruses - spread by email, newspaper, TV etc., infecting
users (causing them to send email to friends and colleagues advising
them of some putative threat, like a new virus)
Boot and binary viruses are largely confined to DOS and
Windows systems, which typically have no protection against
users modifying system files and programs. Unix users are, however,
vulnerable
to Macro viruses and Info viruses.
Precautions
On Windows, install a virus scan program if you download or share software
with other users.
WinTop from the
kerneltoys package
may be useful to monitor system activity.
(wintop needs wintop.exe & wintop.vxd in the same directory; probably
best to explode the exe in a new folder "kernel tools" then put a shortcut
somewhere like programs/system/admin)
Windows Security
See the
Microsoft Security Page
and theWin95 Security Page.
A number of problems have been addressed, including malicious ActiveX
controls, Java applets reading cached passwords, sniffing of NT passwords, etc.