What is a Computer Virus?



A virus can be defined as a computer program which attaches itself to another legitimate program and causes damage to the computer system or network.

What can an adversary do to your computer using a Virus?

One can launch an application level attack or a network level attack using a virus. The program code of a virus runs when the legitimate program it has attached to is executed. It can then infect other programs that are in other computers running on the same network.

Viruses can also be triggered by specific events. Usually most viruses cause damage to computer and network systems to the extent that it can be repaired provided that you had deployed proper data backup and recovery programs beforehand.

Lifetime of a Virus

The lifetime of a Virus can be broken into four phases

  1. Dormant Phase – A virus is idle in this phase. It gets activated based on certain action or event, For example the user typing a certain key or a certain date or time being reached). This is an optional phase in many viruses.
  2. Propagation Phase – In this phase, A virus copies itself and each copy starts creating more copies of self, this propagating the virus.
  3. Triggering Phase – A dormant virus moves into this phase when the action/event for which it was waiting is initiated
  4. Execution Phase – This is the actual work of the virus, which could be harmless like just displaying some message on your screen or destructive like deleting files on your disk.

Classification of Virus

  1. Parasitic Virus – This is the most common form of computer viruses. Such a virus attaches itself to executable files and keeps replicating. Whenever the infected file is executed, the virus looks for other executable files to attach itself and spread.
  2. Memory Resident Virus – This type of virus first attaches itself to an area of the main memory and then infects every executable program that is executed
  3. Boot Sector Virus – This type of virus infects the master boot record (MBR) of the disk and spreads on the disk when the operating system starts booting the computer
  4. Stealth Virus – This virus has intelligence built in, which prevents anti virus software programs from detecting it.
  5. Polymorphic Virus – A virus that keeps changing its signature on every execution making it very difficult to detect is termed as a polymorphic virus.
  6. Metamorphic Virus –  In addition to changing its signature like a Polymorphic Virus, this type of virus keeps rewriting itself, making its detection even harder.

What is a Worm?

A worm is similar in its operation to a virus, because at the end of the day its job is to create problems to the user of the computer. A virus modifies a program, A worm however does not modify a program. Instead it replicates itself again and again. The replication grows so much that ultimately the computer on which the worm resides becomes very slow, finally coming to a halt.

The basic purpose of a worm attack is different from that of a virus. A worm attack attempts to make the computer or network under attack unusable by eating all of its resources.

What is a Trojan Horse?

A Trojan horse can be defined as a hidden piece of code, like a virus. However, The main purpose of a Trojan Horse is different. Whereas the main purpose of a virus is to make some sort of modifications to the target computer or network, a trojan horse attempts to reveal confidential information to an attacker.

The name Trojan Horse is due to Greek soldiers, who hid inside a large hollow horse, which was pulled by Troy citizens unaware of its contents. In a similar fashion, A Trojan horse would silently sit in the code for a Login Screen by attaching itself to it. When the user enters the User ID and Password, The Trojan horse will copy those details and send this data to the attacker.


Your Computer is less prone to get infected by Viruses if you are using a operating system like Linux or macOS. However if you are using Windows, I would highly recommend you to purchase a Anti Virus like this one.



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