The term “boot sector” is a generic name that
seems to originally come from MS-DOS but is
now applied generally to the boot information
used by any operating system. In modern
computers this is usually called the “master boot
record,” and it is the first sector on a partitioned
Boot sector viruses became popular because of
the use of floppy disks to boot a computer. The
widespread usage of the Internet and the death
of the floppy has made other means of virus
transmission more effective.
This type of virus, which can spread itself in numerous ways
including voluntary download, effectively hijacks certain
browser functions, usually in the form of re-directing the user
automatically to particular sites. It’s usually assumed that this
tactic is designed to increase revenue from web
This type of virus, unlike most, only comes into
action when the file containing the virus is
executed. The payload is delivered and then the
virus essentially becomes dormant – it takes no
other action unless an infected file is executed
Most viruses do not use the direct action method
of reproduction simply because it is not prolific,
but viruses of this type have done damage in the
past. The Vienna virus, which briefly threatened
computers in 1988, is one such example of a
direct action virus.
Perhaps the most common type of virus, the file infector
takes root in a host file and then begins its operation
when the file is executed. The virus may completely
overwrite the file that it infects, or may only replace parts
of the file, or may not replace anything but instead re-
write the file so that the virus is executed rather than the
program the user intended.
Although called a “file virus” the definition doesn’t apply
to all viruses in all files generally – for example, the
macro virus below is not referred to by the file virus.
Instead, the definition is usually meant to refer only to
viruses which use an executable file format, such as .exe,
as their host.
A wide variety of programs, including productivity
applications like Microsoft Excel, provide support for
Macros – special actions programmed into the document
using a specific macro programming language.
Unfortunately, this makes it possible for a virus to be
hidden inside a seemingly benign document.
Macro viruses very widely in terms of payload. The most
well known macro virus is probably Melissa, a Word
document supposedly containing the passwords to
pornographic websites. The virus also exploited Word’s
link to Microsoft Outlook in order to automatically email
copies of itself.
While some viruses are happy to spread via one
method or deliver a single payload, Multipartite viruses
want it all. A virus of this type may spread in multiple
ways, and it may take different actions on an infected
computer depending on variables, such as the
operating system installed or the existence of certain
Another jack-of-all-trades, the Polymorphic virus actually
mutates over time or after every execution, changing the
code used to deliver its payload. Alternatively, or in
addition, a Polymorphic virus may guard itself with an
encryption algorithm that automatically alters itself when
certain conditions are met.
The goal of this trickery is evasion. Antivirus programs
often find viruses by the specific code used. Obscuring or
changing the code of a virus can help it avoid detection.
This broad virus definition applies to any virus that inserts
itself into a system’s memory. It then may take any
number of actions and run independently of the file that
was originally infected.
A resident virus can be compared to a direct payload
virus, which does not insert itself into the system’s
memory and therefore only takes action when an infected
file is executed.
Many websites execute complex code in order to provide
interesting content. Displaying online video in your
browser, for example, requires the execution of a specific
code language that provides both the video itself and the
Of course, this code can sometimes be exploited, making
it possible for a virus to infect a computer or take actions
on a computer through a website. Although malicious
sites are sometimes created with purposely infected code,
many such cases of virus exist because of code inserted
into a site without the webmaster’s knowledge.
The threat of viruses infecting our computers is a serious matter. Luckily,
the steps for avoiding an infection on your equipment are relatively simple.
1. The first step in preventing infection is by installing anti-virus software
on your all your computers. A good antivirus system will scan your machine
regularly to clean it from any infected files. Some popular antivirus systems
are Norton Antivirus, Panda Antivirus and McAfee. It is advised to NOT
connect to the Internet without AV protection – the cost is well worth it.
Update your anti-virus software regularly. Over 200 viruses are
discovered each month, so you'll want to be protected.
3. WHEN IN DOUBT, THROW IT OUT! Always use caution and do not
open, download, or execute any suspicious files or email attachments. If the
file was in fact legitimate, you can always ask the sender to resend it. Do not
open any files attached to an email
Delete chain emails and junk email. Do not forward or reply to
any to them. These types of email are considered spam, which is
unsolicited, intrusive mail that clogs up the network.
2. Use caution when downloading files from the Internet. Ensure
that the source is a legitimate and reputable one. Verify that an
anti-virus program checks the files on the download site. If you're
uncertain, don't download the file at all.
3. Back up your files on a regular basis. If a virus destroys your
files, at least you can replace them with your back-up copy.