Information Technology Reference
In-Depth Information
FIGURE 7.2 The attachment to this email message probably contains a virus. (The author
didn't open it to find out.) (Screenshot by Microsoft. Copyright © 2011 by Microsoft Corporation.
Reprinted with permission.)
host program infected with a virus, the virus code executes first. The virus finds another
executable program stored in the computer's file system and replaces the program with
a virus-infected program. After doing this, the virus allows the host program to execute,
which is what the user expected to happen. If the virus does its work quickly enough, the
user may be unaware of the presence of the virus.
Because a virus is attached to a host program, you may find viruses anywhere you
can find program files: hard disks, thumb drives, CD-ROMs, email attachments, and so
on. Viruses can be spread from machine to machine via thumb drives or CDs. They may
also be passed when a person downloads a file from the Internet. Sometimes viruses are
attached to free computer games that people download and install on their computers.
Today many viruses are spread via email attachments (Figure 7.2). We are all famil-
iar with ordinary attachments such as photos, but attachments may also be executable
programs or word processing documents or spreadsheets containing macros, which are
small pieces of executable code. If the user opens an attachment containing a virus, the
virus takes control of the computer, reads the user's email address book, and uses these
addresses to send virus-contaminated emails to others, as illustrated in Figure 7.3.
Some viruses are fairly innocent; they simply replicate. These viruses occupy disk
space and consume CPU time, but the harm they do is relatively minor. Other viruses
are malicious and can cause significant damage to a person's file system.
Commercial antivirus software packages allow computer users to detect and destroy
viruses lurking on their computers. To be most effective, users must keep them up-
to-date by downloading patterns corresponding to the latest viruses from the vendor's
Web site. Unfortunately, many people are negligent about keeping their virus protection
software up-to-date. According to the statistics office of the European Union, a survey
of Internet users revealed that 31 percent of them had experienced a computer virus in
 
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