Absolute PC Security and Privacy.pdf

Chapter 2: How to Catch a Virus
Overview
Everyone can agree that a computer virus is a nasty, destructive thing, and catching a virus is
something to be avoided. But just how do you catch a virus—and how do you know when
you’ve really caught one
While there is general agreement about how viruses are transmitted (and a lot of facts to back
that up), experts don’t always agree about the specific risks involved. For example, is it safe
to surf the Web Can you catch a virus from reading an e-mail message How likely is it that
your computer will be hit by a virus And just how large is the virus threat, anyway
In this chapter we examine all these issues, focusing on what general behavior puts you most
at risk for catching a virus.
How Viruses Spread
Before you can determine what computing behavior you want to risk, you need to know how
viruses are spread from one computer to another.
While the specifics may vary, in general a virus spreads when one computer user receives a
file from another computer user. That file can be delivered on a floppy disk, or downloaded
from the Internet, or attached to an e-mail message—the method of distribution is almost
irrelevant. It’s what you do with that file when you receive it that matters.
• Just receiving the file—saving it to your hard disk—isn’t risky. Your system can’t be
infected just by saving a file. The risk occurs when you open the file. When a program
file (typically with EXE or COM extensions) is opened, the program code loads into
your system’s memory. If there’s a virus in the code, that’s when your system gets
infected.
• When a document file (like a Microsoft Word document) is opened, any macros
attached to the document are run. If there’s a virus in the macro code, that’s when your
system gets infected.
So viruses spread when you receive a program or document file from another user, and then
run or open that file. That’s the activating behavior; it explains why you need to be extremely
carefully when opening files sent to you—by any distribution method.
Of course, there are many ways you can receive files from other users. While all of these
ways of distributing files can spread viruses, some tend to be more risky than others.
Through Infected Media
In the pre-Internet, pre-network days, the only way you could share a file with another user
was to be handed the file—typically on a floppy disk. For that reason, in the early days of the
personal computer era, the most common way of receiving an infected file was by infected
media.