Information About Viruses

What is a virus?
The term "virus" is often used loosely to refer to any one of three major types of rogue programs (Trojan Horses, Worms, and Viruses) that are designed specifically for the purpose of "infecting" a computer without the user's knowledge. Some are intended to erase or damage files, while others simply replicate and spread from computer to computer.

Trojan Horses:
Trojan Horses are deceptive programs that appear to be legitimate on the surface when in actuality, they hide an underlying destructive purpose. For example, an e-mail attachment that appears to be a screensaver program, may display an attractive image or animation upon execution, but while running, such a program may damage important files or otherwise cause undesirable results. Although a Trojan Horse program can cause damage to a user's computer, it cannot replicate and send itself to other computers.

Worms:
Worms are programs that do replicate (make copies of themselves), but do not directly damage files. Worms wreak havoc by reproducing out of control until they use up all of the system resources on the host computer. At this point all tasks are brought to a standstill as the computer is depleted of available memory.

Viruses:
Of the three types of malicious programs discussed here, viruses are usually the most destructive. When a user runs a program that has an embedded or attached virus, the virus is loaded into the computer's memory and then executed. Once executed, it then searches for other programs to infect and performs any destructive tasks it was designed to carry out. Most viruses, described as being "memory resident," remain in the computer's memory even after the originally infected program is closed. This feature allows the virus to continue spreading and causing damage as long as the computer is on.

Because viruses can potentially infect all types of executable code (including the system areas of hard and floppy disks), any removable media can serve as a vehicle for transmission. In addition, files that are attached to e-mail can also carry and transmit viruses. This includes word processing documents that contain macros.

What about e-mail attachments?
It is important to understand that simply reading a plain-text e-mail message will not cause a computer to become infected. Even if the e-mail includes a virus as a file attachment, the user must open and execute the attached program file (or word processing document with macros enabled) for infection to take place*. In short, the only way a computer can acquire a virus is through contact with a file that requires the computer to attempt to run it, or through contact with an infected diskette or similar media that has an executable boot sector. Simple data files such as text, image, or other non-executable files do not pose a threat.

* Note: Recently a worm called VBS.BubbleBoy was devised to take advantage of a security hole in Microsoft Outlook/IE5 e-mail software. This allows the worm to infect a system without the existence of an attached file. The script is actually embedded in the e-mail message and is run when the user reads the message. Visit http://www.symantec.com/avcenter/venc/data/vbs.bubbleboy.html for further information.

What are macros?
Macros are sequences of actions that a user can record or write using a programming language such as Visual Basic for Applications. The macros can then be run to easily play back those actions. Some programs (such as Microsoft Word and Excel) allow users to save documents with embedded macros that can be used by the recipient of that document. If, however, the embedded macros contain a virus, and the recipient opens the document with the macros option enabled, the virus will be transmitted to and stored on that user's computer.

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