Virus is a specific type of malicious program that replicates itself. It can exist as a separate program or as a malicious code attached to other files. While some computer viruses are only limited to replication without other malicious purpose, most of them are dangerous, and some are very destructive.
Due to its behaviour, virus became the first wide-spread and recognized threat to computers. People started to use this specific term for defining any malicious program, and later, term “malware” was introduced to avoid confusion. Though, word “virus” is still used in both ways, and you should understand the difference.
Different Types of Viruses
There are different types of viruses related to some distinct malicious behavior or the method use for its replication. Below some of such types are listed and described:
Virus Classification by Residency
Resident virus loads itself into computer memory in and waits for a time when normal files are executed. Then it infects such files and spread all over the system.
Non-resident virus consists of two components. One seeks for files prone to infection, and another infects these files with a copy of malicious code.
Virus Classification by Behavior
Boot sector virus is a virus that hides in a boot sector on different mass storage disks. It is activated at any time when the operating system is accessing a boot sector of the disk. This kind of virus started to spread since a floppy disks was invented and used for transferring information from one PC to another. It helped to spread earliest viruses, but now this type is relatively rare.
Companion virus attaches itself to the executable files of popular program and ran any time when such program is started. This virus resides in a folder related to the host program and infects specific files used by the host for storage.
Directory virus replaces legitimate system files with its own malicious version that can run legitimate functions just as the original file would do, but they also invoke functions used for some malicious purpose.
Macro virus exploits Visual Basic for Application used for customizing commercial products such MS Office, MS Excel, and any other commercial product that uses VBA for extension. This virus is injected into macros and distributed this way. As soon as the infected macro is started, it infects other files. This type of virus is not destructive to the operating system, but it is potentially viral since saved documents for popular programs are shared very often
Polymorphic virus is designed to fool anti-virus products. It uses encryption for changing itself and infecting PC protected with anti-virus software. After infection it just decrypts itself and runs malicious processes that block anti-virus software or become stealthy. Such viruses are one of the most dangerous types of computer infection because anti-virus software often cannot protect computer against them for a long time needed for discovering such threats and finding effective cure.
This is a limited basic classification related to replicated types of malware called “virus”. Sometimes, we use terms like “redirect virus” or “ransomware virus”, but this is only due to a wide-spread improper terminology that uses specific term “virus” as a replacement for “malware”.
Virus is a specific type of malware that can replicate itself and infect other files. This behavior helps it to infect all components of a computer system or some specific files. Virus can damage your system and hardware devices. Not all malicious programs equals to viruses and the term “virus” is often misused. Anti-virus software is the only protection against viruses.