There are several different types of computer virus. Some examples include:
- The link virus attaches itself to the directory structure of a disk. In this way, the virus is able to manipulate
file and directory information. Link viruses can be difficult to remove since they become embedded within
the affected data. Often, attempts to remove the virus can result in the loss of the data concerned.
- Parasitic viruses insert copies of themselves into legitimate programs, such as operating system files, often
making little effort to disguise their presence. In this way, each time the program file is run, so too is the
virus. Additionally, the majority of viruses are created as terminate and stay resident (TSR) programs. Once
activated, the virus remains in the computer’s memory performing various operations in the background.
Such operations might range from creating additional copies of itself to deleting files on a hard disk.
- Macro viruses are created using the high-level programming languages found in e-mail packages, web
browsers and applications software, such as word processors. Technically, such viruses are extremely crude
but are capable of causing a great deal of damage.
With the possible exception of anti-viruses (described in more detail later), all viruses must be considered to be harmful.
Even if a virus program does nothing more than reproduce itself, it may still cause system crashes and data loss. In many
cases, the damage caused by a computer virus might be accidental, arising merely as the result of poor programming.
There is also evidence to suggest that viruses may be capable of causing physical damage to hardware components. It is
possible, for example, to construct a virus that instructs a disk controller to attempt to read a non-existent track, causing
immediate and irreparable damage to the hard disk drive. Until quite recently, it was thought that computer viruses could
not be attached to data files, such as word processing documents or e-mail messages. However, the built-in programming
languages featured within many modern applications mean that data files may now be used to transmit viruses. However,
it remains true that viruses cannot be transmitted by a conventional e-mail message. A virus can only be transmitted
as an attachment to a message, or if the e-mail package being used allows active content. Two other kinds of programs
are related to computer viruses; worms and Trojans. A worm is a small program that moves through a computer system
randomly changing or overwriting pieces of data as it moves. A Trojan appears as a legitimate program in order to gain
access to a computer system. Trojans are often used as delivery systems for computer viruses.