A computer virus is a malware program that, when executed, replicates by inserting copies of itself (possibly modified) into other computer programs, data files, or the boot sector of the hard drive; when this replication succeeds, the affected areas are then said to be "infected".
Computer viruses are never naturally occurring; they are always man-made. Once created and released, however, their spread is not directly under human control.
Macro Viruses:These viruses infect the files created using some applications or programs that contain macros such as doc, pps, xls and mdb. They automatically infect the files with macros and also templates and documents that are contained in the file. They hide in documents shared through e-mail and networks.
Macro viruses include:
Memory Resident VirusesThey usually fix themselves inside the computer memory. They get activated every time the OS runs and end up infecting other opened files. They hide in RAM.
Memory Resident Viruses Include:
Overwrite VirusesThese types of viruses delete any information in a file they infect, leaving them partially or completely useless once they are infected. Once in the computer, they replaces all the file content but the file size doesn’t change.
Overwrite Viruses Include:
Direct Action VirusesThese viruses mainly replicate or take action once they are executed. When a certain condition is met, the viruses will act by infecting the files in the directory or the folder specified in the AUTOEXEC.BAT. The viruses are generally found in the hard disk’s root directory, but they keep on changing location.
Direct Action Viruses Include:
Directory VirusAlso known as cluster virus or file system virus. They infect the computer’s directory by changing the path indicating file location. They are usually located in the disk but affect the entire directory.
Directory Viruses Include:
Web Scripting VirusMost web pages include some complex codes in order to create an interactive and interesting content. Such a code is often exploited to cause certain undesirable actions. They mostly originate from the infected web pages or browsers.
Web Scripting Viruses Include:
JS.Fortnight - a virus that spreads via malicious emails.
Multipartite VirusThese type of viruses spread in many different ways. Their actions vary depending on the OS installed and presence of certain files. They tend to hide in the computer’s memory but do not infect the hard disk. Multipartite Viruses Include:
FAT VirusesThese lardy viruses attack the file allocation table (FAT) which is the disc part used to store every information about the available space, location of files, unusable space etc.
FAT Viruses Include:
the link virus
Companion VirusesThese types of viruses infect files just like the direct action and the resident types. Once inside the computer, they ‘accompany’ other existing files.
Companion Viruses Include:
stator and terrax.1069
Polymorphic VirusThey encode or encrypt themselves in a different way every time they infect your computer. They use different encryption and algorithms. This makes it difficult for the antivirus software to locate them using signature or string searches (since they are very different in each encryption).
Polymorphic Viruses Include:
WormThis program is very similar to a virus and has the ability to self-replicate leading to negative effects on your computer.
Worm Viruses Include:
TrojansTrojans can illegally trace important login details of users online. For example E-Banking is very common among users, therefore, vulnerability of tracing your login details whenever your PC is working without any strong powerful antivirus installed.
Email VirusThis is a virus spread via an email. Such a virus will hide in an email and when the recipient opens the mail.
Browser HijackerThis virus can spread in many different ways including a voluntary download. If infects certain browser functions especially in form of re-directing the user automatically to certain sites. A good example is
Browser Hijackers Include:
the cool web search
Boot InfectorsThey include the boot sector plus master boot record types. All the viral codes can be separate location; however they infect the hard disks or the floppy.
Boot Infectors Include:
the brain virus -it is the very first wild virus to be created.
- First actual virus - Creeper (1970s),detected on ARPANET. It displayed a message on the screen, "I am the creeper, catch me if you can!"
- First actual personal computer virus - Elk Cloner (1982), it affected Apple OS through a floppy and displayed a short poem!
- First virus to hit Microsoft's DOS - Brain (1986)
- Most destructive virus - MYDoom (January 2004)
- Most widespread and fast spreading virus - ILOVEYOU, also known as the love letter! (2000)