What is a virus? Computer viruses are programs that disseminate similar to biological viruses and after becoming registered inside the computer, they perform unexpected actions. Even though not all viruses are considered to be dangerous, many of them are written with the intent to infect all kinds of personal files, software applications and operative systems. Viruses similar to many other applications benefit from sources such as memory, hard drive storage, and Central Processing Unit (CPU); further, viruses are capable of performing dangerous tasks like deleting files from the hard drive and reformatting the entire hard drive. Also, a virus can provide justification with respect to accessing the computer system through the network and without capturing personal information. For the first time in 1984, the term "virus" was invented by the pioneer Fred Cohen and was used in many academic contexts. In his 1984 paper called "Experiments with Computer Viruses", Fred has credited Leonard Adleman with coining the term “virus”. Indeed, before this period, viruses loomed in practical and imaginative contexts. The primary boot sector virus is a kind of virus that is mounted on a floppy disc and the hard drive. In these sectors executive codes are stored to make the computer run and operate. Considering the fact that each time the computer is booted, the boot sector is being assigned, and with each time a change is bound to occur inside the computer, the entire content of the boot sector gets to be rewritten, because these local sectors can pose threat against virus attacks. These types of viruses spread themselves to those floppy discs that already contain infected boot sectors. The boot sector of the hard drive becomes infected as a result of an infected virus; each time that the computer is booted, the computer loads the infected virus and stores it in its own memory and the virus awaits an opportunity to infect the floppies until it is capable of spreading itself further and infecting the remainder of the entire computer system. When an operative system is infected, these types of viruses are capable of effacing the boot sector of the hard drive and wiping the slate clean. After writing themselves to the boot sector, these boot sector viruses make the attempt to spread themselves to other sectors that are considered to be bad sectors. Macro viruses are computer viruses that do not directly infect programs; macro viruses focus on documents that are created as a result of programs that use an application’s own macro programming language like Word or Excel. Macro viruses distribute themselves to drives, network, and files that are enclosed in electronic mail message delivery application. A macro virus becomes activated when an infected document is opened in the word processor; in this case, the virus begins to distribute itself and infect the rest of the executive files of the system. Some of these kinds of viruses alter data; they may add themselves to existing executable files. Polymorphic viruses appear differently in each single infected file and make use of coded algorithms and wipe their malignancies. The distinction and identification of polymorphic viruses is a difficult task. Hidden viruses try to hide themselves and keep away from their governing system and anti-virus software applications. For this purpose, the virus resides in memory and blocks its access to the governing system. In this case, the virus is the recipient of all the requests the anti-virus software application sends to the governing system. In this manner, anti-virus software applications become deceived and yet lead us to believe that no more viruses reside inside the computer. Current in vogue, boot and the file-infecting viruses are compound of the multipartite or multi-part viruses; the synthesis of other viruses is also possible. Many researchers do not categorize destructive programs suchas trojan horses, worms and eliminative bombs as viruses, but reality proves that such malware can pose serious threat and danger to the entire computer system. Trojan horses may claim to be performing specific tasks but in practice they are designed for some other purpose. For example, a program may claim that it is a computer game but in reality provides remote access to the computer. Worms are programs that similar to viruses are capable of replicating themselves; worms often make use of harmful e-mail applications for fast and wide distribution. Eliminative bombs are programs that are predetermined and meant to perform unexpected actions.
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