by Torbj√∂rn Hovmark, president of Abtrusion Security AB
Originally, the term virus was used for a particular type of
software that could infect other programs. Today we normally use the term
much more loosely to describe any kind of software program that spreads more
or less by itself.
A virus can take many forms. There are the Trojans that
hide within other, seemingly useful programs. There are the macro viruses
that will infect documents - typically Office documents, as everyone has
some version of Microsoft Office installed these days. There are the worms
and mail viruses, mutating viruses and many other types.
Once upon a time, viruses spread from computer to computer by means of
floppy disks. These were the days of the infecting viruses, as they would
have to sneak along with another program to spread. At that time, Macintosh
computers were especially hard hit. A Macintosh floppy disk includes a piece
of code that is automatically run whenever the disk is inserted into a
Then came bulletin boards, download areas and CompuServe. Virus makers
found new ways of spreading their creations by infecting software for
download. This all culminated with the advent of the Internet.
Along with the Internet came large scale use of e-mail. Along with e-mail
came e-mail worms. They use the e-mail system to spread and have proven to be
the most effective creatures invented by the virus makers so far.
Recently, a couple of viruses using regular hacker techniques have also
shown up in the wilderness out there on the Internet. They hack into web
servers and then spread to the web site's visitors or to other web servers.