Kreća Marija 275/07
Computer virus is a program that can
reproduce itself in that way adding its own
code to another program.
The result of virus perform can be writing
some note on a monitor, or displaying some
picture, modifying or erasing some files and
In other words, virus can do everything that
any other program does.
Fred Cohen, an expert for viruses, developed
mathematical model of virus behavior.
His formal definition for computer viruses is:
“Computer virus is a computer program that can infect
other programs by modifying them and in that way
makes copies of itself.”
The virus author’s intentions are:
Quick virus expanding
Difficult virus detecting
Complicate virus defense.
Translation of a program which contains virus on our
language would be:
Load yourself into the memory (in that way it will be
activate until you shut your computer down)
Wait for starting some other program
While you wait, disrupt the keyboard
After starting some other program insert this whole
program in it.
Of course, there are also kinds of viruses that don’t wait
for execution of other program to infect it, but they search
throw the disc to find an EXE file and to infect it.
Warning Signs of Virus Infection
1. Your computer starts running sluggishly
2. It shuts down unexpectedly
3. It experiences memory problems or runs out of disc
4. Unusual files or directories appear on your system
5. Strange messages appear on your screen.
How Does It Happen?
For example, your friend’s computer is infected
and that friend brought you some game on CD. He
burned that game on his computer and it is most
likely that the CD is infected. When you start the
game on your computer, the virus enters in the
memory of your PC. After your game playing, you
start some other programs and the virus infects
them, too. So, the virus will in just few days infect
all of the programs on your computer. If you give
someone else some program from your infected
computer you will expand the virus.
These programs are very serious things and it can be a
real life threat. In case of some hospital’s computer
system, virus can stop the computer and display some
innocent message on the screen. Computer viruses are
a special case of so-called “malicious logic” or
First type of viruses was funny, interesting because
they were displaying some amusing message and they
weren’t harmful. But, things have changed. In the 1980
computer viruses were parts of a code in the program,
so they were executing when the program starts.
Today, authors are familiar with some new tricks.
Some of them would be a possibility of writing the
virus into the memory, so it can run while the
computer is turned on.
Of course, there is a protection, or so-called antivirus
programs, that protect the boot sectors of a computer.
There are several types of computer viruses:
Boot sector viruses - infect the boot sector of a floppy or
File infector – can be very infectious and it is harder to
recognize it than the boot sector virus.
Macro viruses – small programs written in some inside
code of some application program such as Word, Excel…
Internet viruses – parts of e-mail and they can multiply
themselves from addresses. The most famous virus was
the Melissa from 1999. After that, the I love you virus
appears in 2000.
Today, we can tell that there are some categories of viruses:
Hoax – the virus that warns us about supposed appearing
some very serious virus.
Joke – harmless program that causes some harmless
Worm – program that infects other programs, boot sector and
a document that supports macros. It can be very harmful
because it can replicate itself. Usually the computer has to be
connected to the Internet to get a worm.
Trojan horse – an application that can make a lot of damage
to your computer spreading like virus. It can allow someone
else to operate on your computer sending him your
passwords. Trojan horse can infect your files by entering from
Antivirus programs use two accesses for detecting a
virus: scanning and heuristic. When scans, it can
detect the lines of a code of a virus or some of its
variation. Heuristic is a method of searching some
When you work over the Internet, there is a possibility
that the virus enters into your computer before you get
chance to check the files.
Making a reserve copies is very useful, and it’s some
way of protection.
Six Gold Steps in Using Antivirus
1. You must install an antivirus program
2. Regular download updates for your antivirus
3. Set your antivirus program automatic scan your
4. Scan all of the files that arrives from the Internet
5. Scan your whole disc from time to time
6. Scan hard disc after software installation.
Thank your for your attention!