A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The original virus may modify the copies, or the copies may modify themselves, as occurs in a metamorphic virus . A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or the Internet, or by carrying it on a removable medium such as a floppy disk , CD , or USB drive . Additionally, viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with computer worms and Trojan horses . A worm can spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a file that appears harmless until executed.
Many personal computers are now connected to the Internet and to local area networks , facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware .
Some viruses are programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Others are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these benign viruses can create problems for the computer user . They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, many viruses are bug-ridden, and these bugs may lead to system crashes and data loss.
Worms are a sub-category of virus that technically causes no harm to your computer. The majority of worms copy and send themselves through your email or network to everyone possible. Unlike viruses, they do not need to be opened, and can spread without any human intervention. They can cause a lot of damage because these exponential replications can quickly crash a network if not stopped. Some worms also carry a payload, defined as a damaging program directed at the computer itself. An example is the Blaster Worm, which not only caused networks to crash, but also severely hampered individual computers ability to access the Internet.
Trojan horses, like the story from which they come, are stealth viruses. A Trojan will typically enter your system by disguising itself as a friendly program until executed, whereupon it will become malicious. Unlike viruses and worms, the Trojans cannot replicate themselves and must be spread through the exchange of the programs to which they are attached. They attack computers rather than networks, and have been known to do everything from deleting files to creating backdoors, points of easy access which allows hackers to access the files on your computer remotely. An example of a Trojan horse is SpySheriff, which masquerades as an anti-spyware program but when executed essentially takes over a user's computer, preventing access to files and the internet. With the threat of new malware being a constant concern, it can be hard to know how to protect your computer. If you have good firewall and anti virus software you can be protected from these external threats.
