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Cyber 911 Tiplines Computer viruses, worms and Trojans
What they are and what they can do
Computer viruses are self-executing, replicating programs written specifically to change the way a machine works, without the knowledge (or permission) of the operator/owner. Viruses are so called because they behave in a similar way to biological viruses. Just as biological viruses pass from person to person, replicating themselves as they go, computer viruses pass from computer to computer. Unlike most biological viruses, however - computer viruses are entirely man made.
Viruses can impair and seriously damage your computer (or network server) by, amongst other things; executing random text, audio and video messages, draining memory, deleting files, corrupting programs - even reformatting (erasing the contents of) your hard disk. At best, the less destructive variants are irritating and will slow the infected machine up (because of the drain on memory), often resulting in crashes and other unpredictable behaviour which can ultimately result in loss of data.
Although a virus needs an infected application to be launched in order to infect other programs or documents, they can conceal themselves in your computer (often masquerading as innocent files) and replicate (make copies of) themselves until such an infected application is launched.
Not all viruses behave in precisely the same way and not all malicious programs are viruses (like Trojans). Some viruses are only active whilst an infected applications is running, whilst others will stay active in memory until you turn off your computer. However, as the virus is resident in a file or on a disk, exiting the infected application or turning off your computer only removes the virus from memory, it does not remove the virus from the infected file or disk and the virus just lays dormant, until you to reboot your computer and/or access the infected application.
The various forms of computer virusesBoot sector viruses infect the boot sector of a hard drive or floppy disk by first overwriting/moving the original boot code and then moving the original code to another sector on the disk, which the virus marks as bad.
File infecting viruses attach/modify any executable files, sometimes replacing the original code with its own.
Macro viruses are self replicating macros that self replicate and can spread rapidly on a computer and/or network.
Master Boot Record Infectors infect a system's Master Boot Record on hard drives and the Boot Sector on floppy diskettes.
Multi-partite viruses are commonly a combination of techniques of both boot sector viruses and file infecting viruses.
Polymorphic viruses are difficult to detect as they use an encryption algorithm that changes, along with the viruses' appearance, change their appearance with/after each infection.
Stealth viruses hide themselves from a computers' operating system and anti-virus products.
Viruses (including worms) are often distributed via attachments in e-mail spam and, ironically, a great deal of e-mail spam (particularly chain letters) are virus hoaxes.