Information Technology Reference
In-Depth Information
unlikely to consciously ask the CPU to run a virus. Instead, viruses
normally come packaged as a behindthescenes part of something
else, and when the other item is activated by the computer, the virus
also is activated. Here are some possible scenarios in which a user
might accidentally encounter a virus and its work:
A virus can be received as an email attachment. In the form
of an attachment, the virus is harmless, because email will
just treat it as data. However, if the user opens the attach
ment, and if the user's email handler allows the execution of
opened attachments, then the act of opening the attachment
instructs the computer to run the program, allowing the virus
to begin its processing.
A virus can insert itself into another program. (In this case,
the virus is said to have infected the other program.) When
the user runs the other program, the virus also is activated.
A virus can be part of the formatting instructions for a word
processing file or part of a computation for a spreadsheet. In
this case, the virus is part of the user's data file for an appli
cation, and the virus is activated when the infected part of the
data is accessed.
A virus can also be embedded as a program within a Web page.
As we will see in later chapters, a Web browser can be set to
run programs. Often, programs running on a Web page are
useful. For example, they may be intended to display some
helpful material or to analyze data as they are typed into the
Web page. However, browserbased programs can also open
and be destructive, in which case they would be classified as
viruses. (In principle, a browser is supposed to prevent various
undesirable activities from taking place, but errors in the
browser program itself might allow other, lessdesirable activi
ties to take place.)
A virus can be inserted into a disk, in a place that is routinely
activated when a computer first starts up. In this case, start
ing the computer would also execute the virus.
Although many of these scenarios seem to involve rare events,
like receiving an infected email, opening a Web page that contains
a virus, or reading an infected disk, a virus often includes a compo
nent that helps it expand the number of files it infects. For example,
