Virus

The term 'computer virus' was first formally defined by Fred Cohen in 1983. Computer viruses never occur naturally. They are always induced by people. Once created and released, however, their diffusion is not directly under human control. After entering a computer, a virus attaches itself to another program in such a way that execution of the host program triggers the action of the virus simultaneously. It can self-replicate, inserting itself onto other programs or files, infecting them in the process. Not all computer viruses are destructive though. However, most of them perform actions that are malicious in nature, such as destroying data. Some viruses wreak havoc as soon as their code is executed, while others lie dormant until a particular event (as programmed) gets initiated, that causes their code to run in the computer. Viruses spread when the software or documents they get attached to are transferred from one computer to another using a network, a disk, file sharing methods, or through infected e-mail attachments. Some viruses use different stealth strategies to avoid their detection from anti-virus software. For example, some can infect files without increasing their sizes, while others try to evade detection by killing the tasks associated with the antivirus software before they can be detected. Some old viruses make sure that the "last modified" date of a host file stays the same when they infect the file.Description: It fits the description of a computer virus in many ways. For example, it can also self-replicate itself and spread across networks. That is why worms are often referred to as viruses also. But computer worms are different from computer viruses in certain aspects. First, unlike viruses which need to cling on to files (host files) before they can diffuse themselves inside a computer, worms exist as separate entities or standalone software. They do not need host files or programs. Secondly, unlike viruses, worms do not alter files but reside in active memory and duplicate themselves. Worms use parts of the operating system that are automatic and usually invisible to the user. Their existence in the system becomes apparent only when their uncontrolled replication consumes system resources, slowing or halting other tasks in the process. In order to spread, worms either exploit the vulnerability of the target system or use some kind of social engineering method to trick users into executing them. Once they enter a system, they take advantage of file-transport or information-transport features in the system that allows them to travel unaided. A computer worm called 'Stuxnet worm’ turned heads the world over recently when it attacked the nuclear facilities of Iran. This worm reportedly destroyed roughly a fifth of Iran's nuclear centrifuges by causing them to spin out of control by increasing the pressure on the spinning centrifuges, while displaying that everything was under control. It managed this feat by replaying the plant's protection system values in the control room while the attack was happening.