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How Computer Viruses Work?

Updated on October 22, 2011

A worm has the ability to copy itself from one computer to another. It is a simple computer program, which moves around through computer networks.

In the late 1980s, computer viruses were born. It was a time when games were very popular, along with simple word processors and spreadsheets. Users would dial up a bulletin board with a modem to download different and new types of games. This process brought in the virus named Trojan horse.

Trojans were used to get into the computer with a program or a game, and when users ran the program or game on their computer, the Trojan would usually erase the entire disk in the system. Trojans did not damage many computers because they were discovered quickly. Back then, programs were small and the entire operating system could fit into a floppy disk or two. It was because of this that floppy disks played a tremendous role in giving rise to viruses.

Computer viruses have always been intriguing, for they wreak utter chaos in one's system, without making their presence felt initially. However, not all viruses have managed to steal the limelight. One virus, which was notorious for its destruction, was the Mydoom worm. This virus infected approximately a quarter-million computers in one day. Then there was the Melissa virus, which was so powerful that it forced Microsoft and several other companies to completely turn off their email systems until the virus could be cleaned. The "I Love You" virus had a similar effect on email systems.

Viruses can be categorised as follows:

  1. The basic virus: a small piece of software that rides on real programs. A virus might attach itself to a program, such as a spreadsheet program. So, whenever that program runs, the virus will run along with it and will then reproduce, by attaching itself to other programs.
  2. Email viruses: they move around in email messages, replicating by automatically mailing themselves to dozens of people at one time. Email viruses usually come with a message that says you have won thousands of dollars, or provides so-called never-before-seen footage of celebrities, etc.
  3. Worms: small pieces of software that use computer networks and security loop holes to replicate themselves. A worm scans the network for another machine that has the same security loop hole, copying itself to the new machine using the security loop hole. From here it starts to replicate.
  4. Trojans: a very simple computer program, Trojans are usually games. When you run the game it erases your hard disk as you play the game. Trojan horses do not replicate themselves automatically.

Why are computer viruses called “viruses”?

It is because they are the same as biological viruses. Biological viruses pass from person-to-person, while computer viruses pass from computer-to-computer. A biological virus is a fragment of DNA inside a protective jacket and cannot reproduce itself as such. Instead, it injects its DNA into a cell, so that it can reproduce itself. A computer virus does the same, sitting on top of some program and document in order to get executed, and once the program or document runs, it infects other programs and documents.

A worm has the ability to copy itself from one computer to another. It is a simple computer program, which moves around through computer networks. Worms replicate very quickly; for example, the Code Red worm replicated itself 250,000 times in approximately nine hours. A worm usually makes its way into an operating system through a security loop hole. In January 2003, a worm, called the Slammer, made a hole in Microsoft's SQL Server, which damaged the server.

Similarly, Code Red replicated itself and slowed down internet traffic. It scanned the internet for Windows NT and Windows 2000 servers that did not have the Microsoft security patch installed and whenever it found a vulnerable server, it copied itself to that server, infecting it. This replication process continued and internet traffic slowed down to a great extent.

Code Red worm's main purpose was to successfully infect the White House domain. The attack would consist of sending 100 connections simultaneously to port 80 of But the US government changed the IP address of so that it could not be harmed by the Code Red worm. It also issued a general warning about the worm to the users of Windows NT and Windows 2000 server, and told them to install the security patch.

Executable viruses

These viruses were simple and were small pieces of code attached to the most common program which could be a popular game or word processor. A user could download that game or word processor from the bulletin board and then run it on their system. These viruses are designed to run first when the legitimate program is executed. The virus then loads itself into the memory and looks around to see if it can find any other programs on the disk. If it finds a program it adds the virus code to that program. If a user has an infected program and gives it to anyone on a floppy disk (or a CD), or uploads that program on the bulletin board, then the virus spreads and infects other programs.

Boot sector viruses

The boot sector is a small program that is the first part of the operating system a computer loads. It also contains a tiny program that tells the computer how to load the rest of the OS. If the virus code enters the boot sector, then the virus is executed. Both, boot sector viruses and executable viruses are not really a threat anymore, because programs today are huge in size compared to the ones before, and nearly every program now comes on a CD, which cannot be modified, making infection impossible. Also, the OS now protects the boot sector, making these viruses useless.

Email viruses

Email viruses are the latest thing in the world of computer viruses. An email virus, known as the Melissa virus, born in March 1999, was quite powerful. Created as a word document, Melissa was uploaded to an internet news group. Anyone who downloaded the document and opened it ran the risk of infecting his/her system. Then, the virus sent the same document to the first 50 people in the person's address book.The email message contained a friendly note that included the person's name, so the recipient would think it was from someone they know and so deemed the email harmless. The virus continued doing this from every machine it hit and was the fastest spreading virus ever seen. In fact, it forced a number of large companies to shut down their email systems. Another email virus that appeared on May 4, 2000, was known as the "I Love You" virus. This contained a piece of code with an attachment. Anyone who opened the attachment opened the virus and it then sent copies of itself to everyone in the person's address book. After that, it started corrupting the user's machine.

Viruses are not the work of nature and neither have they come from another planet so they are not aliens. Viruses are a product of the human mind. People who invent or create viruses are not stupid, but, in fact, quite intelligent. But these brilliant minds have become corrupt. Perhaps they do this just for fun, or for revenge.

Whatever the reason, the fact of the matter is that viruses are troublesome, because they cost people time and money. I personally think that if these people use their skills in a positive manner, then they would probably be the most successful people alive.


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    • profile image


      6 years ago

      great information

    • profile image


      7 years ago

      thanx this was helpful

    • profile image


      7 years ago

      excellent information


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