By definition, a virus program is able to replicate itself. This means that the virus multiplies on a computer by making copies of itself. This replication is intentional; it is part of the virus program. In most cases, if a file that contains virus is executed or copied onto another computer, then that computer will also be "infected" by the same virus.
A virus can be introduced to a computer system along with any software program. For Internet users, this threat can come from downloading files through FTP (file transfer protocol), or referencing email attachments. (Please refer to our web page on Handling Email's File Attachments for details.)
When a virus is introduced to a computer system, it can attach itself to, or sometimes even replace, an existing program. Thus, when the user runs the program in question, the virus is also executed. This usually happens without the user being aware of it.
A virus program contains instructions to initiate some sort of "event" that affects the infected computer. Each virus has an unique event associated with it. These events and their effects can range from harmless to devastating. For examples:
- An annoying message appearing on the computer screen.
- Reduced memory or disk space.
- Modification of data.
- Files overwritten or damaged.
- Hard drive erased.
Types of Viruses
- File virus : Most viruses fall into this category. A virus attaches itself to a file, usually a program file.
- Boot sector virus : These viruses infect floppy and hard drives. The virus program will load first, before the operating system.
- Macro Virus : This is a new type of virus that use an application's own macro programming feature to distribute themselves. Unlike other viruses, macro viruses do not infect programs; they infect documents.
- Virus Hoax : Although there are thousands of viruses discovered each year, there are still some that only exist in the imaginations of the public and the press - known as virus hoaxes. These viruses hoaxes DO NOT EXIST, despite rumor of their creation and distribution.
Do not pass on Virus Hoaxes
Although there are thousands of viruses discovered each year, there are still some that only exist in the imaginations of the public and the press - known as virus hoaxes. These viruses hoaxes DO NOT EXIST, despite rumor of their creation and distribution.Please ignore any messages regarding these supposed "viruses" and DO NOT pass on any messages about them. Send them to ITSC to be dealt with. Passing on messages about these hoaxes only serves to further propagate them.
Trojan Horse is Not Virus!
Strickly speaking, a trojan horse is NOT a virus because it does not replicate like ordinary viruses do. A trojan horse is an unfriendly program which will appear to be something other than what it is, for example a program that is disguised as a legitimate software program.
Below are some common trojans:- Back Orifice 2000 : BO2K allows outsiders to access and modify any information on a Windows 95, 98 and NT machines through an invisible server program installed by the program. It also allows outsiders to spy on what user is doing. BO2K is expected to be released on 10/7/1999. Users are advised not to run executables from un-secured source and updates their antivirus tools periodically. Details can be found at http://ntbugtraq.ntadvice.com/bo2000.asp
- Back Orifice: This trojan horse allows an intruder to monitor and tamper with Windows 95 and Windows 98 computers over the Internet. There is no easy way for a computer user to know the attack is taking place, and there is no easy way to stop the attack once Back Orifice has installed itself on the computer. The ver 1.2 of Back Orifice cannot be installed on Windows NT system.
- NetBus: NetBus is a remote administration tool, much like the infamous Back Orifice tool. However, Netbus predates Back Orifice by several months and is also capable of working under Windows NT in addition to Windows 95 and 98.
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