A virus is a piece of code that is able to copy itself and is capable of destroying the data saved and downloaded in your computer. It is a malicious software that replicates by modifying computer programs, replacing it with its own code. When the replication is complete, the areas damaged by the code are said to be infected. The writers of such code use deception to exploit the knowledge about vulnerabilities in the security system to spread the virus.

They use many types of mechanisms to infect. They use a complex system of anti-detection and stealth strategies to evade the antivirus. Viruses are made to threaten, seek profit, deliver a message, cause sabotage, deny a service, and explore cyber security, among other motives.

Virus causes billions of dollars of economic damage due to system failure, corrupt data, maintenance costs, and wastage of computer resources. To counter the viruses of various types, there are antivirus tools and software for different operating systems.

Computer security researchers have developed antiviruses that help detect new viruses before they get widely spread. The term “virus” was used to refer to malware, which also refers to malicious software like worms, spyware, adware, ransomware, Trojan, rootkit, keyloggers, and root kits.

Active malware threats include Trojan horses and worms, but not viruses. Viruses infect the computer like hard disks and CPUs to access private information such as bank account details, credit card information, personal data, and email contacts and to render the computer useless. Not every type of virus is destructive.

A viable computer virus has a search routine that it uses to locate new files and disks to infect. Furthermore, it must have a routine to copy itself.

A virus consists of three main sections:

• The infection mechanism is used to spread the virus. It has a search routine for locating new files or new disks for infection.
• The trigger is a bomb that is activated with an executable file. The file determines the event where the payload will be activated–date, time, and file.
• Payload is the actual data body that will perform the malicious act. The payload may create a reaction in the system. It slows down or freezes the system which in itself may be harmful or just a hoax.

A virus has the following phases in its life cycle:

• The Dormant phase is when the virus does not act in any way. It will only be triggered at a date, time in the presence of a file.
• The Propagation phase is when the virus is replicating itself. It morphs to avoid detection. Each infected program is a clone of the virus.
• The Triggering phase when the virus is activated and will perform the actions it was intended to do. Triggering can be due to a system event.
• The Execution phase is the actual phase when the payload is released causing destruction, deleting files, corrupting files, crashing the system, popping messages, etc.

Types of viruses
This includes binary executables that are .com or .exe files, word files or PDF files, and they can be in the boot sector and hard drive.

• Residents and non-resident viruses
  A memory-resident virus installs itself as a part of the OS and remains on the RAM until the system is booted. These interrupt the handling code and other functions when the OS tries to access the file redirects the flow of the control. When a non-memory-resident virus is run, it scans the OS, infects it, and then departs.
• Macro virus
  Many applications like Microsoft Word and Outlook allow the embedding of macro programs in emails and documents. They are automatically executed when one clicks “run.” This is written in a macro language and embedded in viruses. When one opens the file, the code is executed and infect the computer. Not opening the attachment with emails reduces the chances of getting affected by a virus.
• Boot sector virus
  Apart from the boot sector, this virus targets removable storage, hard drives, etc.
• Email virus
  An email virus is spread through emails and attachments and it targets specific files in the system.