Computer viruses are small pieces of program code at sometimes lock on to the boot sector of the computer system they can Replicate itself as it spreads around the system and causes damage.
This is what the website how stuff works descries them found at http://www.howstuffworks.com/virus.htm
Computer viruses tend to grab our attention. On the one hand, viruses show us how vulnerable we are. A properly engineered virus can have an amazing effect on the worldwide Internet. On the other hand, they show how sophisticated and interconnected human beings have become.
For example, experts estimate that the Mydoom worm infected approximately a quarter-million computers in a single day in January 2004. (Times Online). Back in March 1999, the Melissa virus was so powerful that it forced Microsoft and a number of other very large companies to completely turn off their e-mail systems until the virus could be contained. The ILOVEYOU virus in 2000 had a similarly devastating effect. That's pretty impressive when you consider that the Melissa and ILOVEYOU viruses are incredibly simple.
When you listen to the news, you hear about many different forms of electronic infection. The most common are:
Viruses - A virus is a small piece of software that piggybacks on real programs. For example, a virus might attach itself to a program such as a spreadsheet program. Each time the spreadsheet program runs, the virus runs, too, and it has the chance to reproduce (by attaching to other programs) or wreak havoc.
E-mail viruses - An e-mail virus moves around in e-mail messages, and usually replicates itself by automatically mailing itself to dozens of people in the victim's e-mail address book.
- A worm is a small piece of software that uses computer networks and security holes to replicate itself. A copy of the worm scans the network for another machine that has a specific security hole. It copies itself to the new machine using the security hole, and then starts replicating from there, as well. Worms
Trojan horses - A Trojan horse is simply a computer program. The program claims to do one thing (it may claim to be a game) but instead does damage when you run it (it may erase your hard disk). Trojan horses have no way to replicate automatically.
“The computer security resource center (CSRC) says this about computer viruses found at http://csrc.nist.gov/virus/
so what you can do use a virus checker, use firewall protectionViruses are the colds and flues of computer security: ubiquitous, at times impossible to avoid despite the best efforts and often very costly to an organization's productivity.
NIST recommends using a two-tiered approach for detecting and preventing viruses from spreading:
On personal computers, install and use anti-virus software capable of scanning disks, attachments to email, files downloaded from the web, and documents generated by word processing and spreadsheet programs.
Use anti-virus software at Internet gateways or firewalls to scan email attachments and other downloaded files.
Anti-virus software should be installed when the personal computer is initially configured. The software should be updated weekly with new virus definitions, and your vendor may provide an automated update feature. Organizations may benefit from using several brands of anti-virus software.