Virus
A program or programming code that replicates by being copied from one computer to another, usually through a shared document or file. Viruses are a form of malware.
Viruses can be transmitted as attachments to an e-mail note, in a downloaded file, or on a diskette or CD or DVD. Some viruses wreak havoc as soon as their code is executed while others lie dormant until automatically executed by a pre-determined trigger. Viruses can range from benign to malicious, sometimes destroy your data on your hard disk.
A virus that replicates itself by resending
itself as an e-mail attachment is known as a worm. A worm does not
alter files but resides in active memory and just duplicates itself.
Worms use parts of an operating system that are automatic and
usually invisible to the user, and are only detected when
replication consumes system resources and slows your computer down.
Macro viruses are among the most common viruses but generally they
tend to do the least damage. They infect Microsoft Word applications
and typically insert unwanted words or phrases.
The best protection against a virus is to know the origin of each
program or file before you load it into your computer or open it
from your e-mail. Additionally, use at least one anti-virus software
and updated it regularly.