Every since the dawn of computing and users started to write their own software there has been there have been viruses however it wasn't until the 1980s that the potential threat of computer viruses became common knowledge. The action of a virus is not too dissimilar to that of its name, it is a small computer program that makes its way onto a computer with the aim of duplicating itself and infecting other computer systems. Although a virus doesn't have to have bad intentions it is unfortunate that almost all of them have.
In the past viruses would spread by hiding within files downloaded from bulletin boards or on floppy disks used to store and transfer documents. As computer technology advanced the introduction of computer networks, the internet and email provided ample opportunities for a virus to spread amongst systems. Initially a virus makes its way onto a computer in the form of a trojan.
A trojan, taken from the term trojan horse, is essentially a file or program that appears appealing enough for somebody to download it or execute it. Hidden within this file however is a virus which once activated spreads onto the host computer system and hides. Once it has infected the system the virus can set about spreading amongst files and applications and can even utilise existing network and internet connections to reach as many other computers as possible.
Virus Types:
Although countless viruses exist they can generally be categorised into one of the following:
Standard Virus - As mentioned above a virus is a small piece of code that infects an application and is executed each time that program is used. A popular method of viruses to ensure activation was to attach themselves to the boot sector of a floppy disk or hard disk. The boot sector contains important information regarding the disk and is executed when a computer first accesses the disk. By hiding here the virus is guaranteed to be activated each time the computer boots up or each time an infected floppy disk is used. Viruses also infect popular applications such as word processors, the result is that each time the application is used the virus is running and a clever virus will attach itself to documents that are saved and will spread to whomever next reads those files.
Email viruses - An emails virus is one which attaches itself to email messages either in the form of some hidden code in the body of mail which is executed when the email is read or as an attachment that is linked to the email - note that attachments can also be used as trojan files! Email viruses are particularly clever as they automatically attach themselves to every outgoing email that is sent and some also have the capability of sending themselves to all contacts in the users address book!
Worms - A worm is similar to a virus in as much as it is again a small piece of software however a worm will utilise a computer network by looking for security holes in the setup or the software that is being used. Worms tend to be written to take advantage of known bugs or secuirty issues in popular software packages and force the software developers to quickly release a security patch to rectify the fault. Once a worm has found its way into a system or computer network it duplicates itself and then continues to utilise the same security holes to spread further!
Trojan Horses - As mentioned earlier a trojan horse is a piece of code which looks to be a normal computer application, and is usually one which will make the user want to download and install it. However once the 'fake' program is run the trojan code is executed and does its damage which may, in extreme cases, be to erase the contents of the hard disk it is being executed from. Trojans do not usually have the ability to replicate unless they have a virus or worm hidden within them also.
Stephen is a Cisco qualified network administrator for a leading University and is currently studying towards an MSc in Telelecommunications. 