A virus is a program that attaches itself to other programs, so that when users to run the original program they run the virus code as well. The virus code is designed to replicate itself and infect other programs and may also cause damage to systems or data.
Viruses have existed for almost as long as PCs, but in recent years they have become a major problem, largely due to the number of different ways in which they can infect a computer: they can be downloaded from the Internet, transferred over networks or be picked up from removable storage such as floppy disks, CD-ROMs or USB drives.
Some writers have suggested that Windows-based PCs are more susceptible to viruses than Unix/Linux or Apple Macintosh machines, but others believe that they simply offer a bigger target and are thus more attractive to virus writers. There has been a noticeable increase recently of attacks on Unix/Linux machines used as Web Servers.
The behaviour of computer viruses is remarkably similar to that of biological viruses and techniques for dealing with them show similarities as well. Biological viruses reproduce rapidly in order to spread to other hosts and perpetuate themselves. They move from host to host and may return to re-infect the original host at a later date. Computer viruses are similar. They spread from PC to PC and may return to re-infect the original host.
There are other types of malicious programs (malware) that are similar to viruses in terms of how they work and what they do, but differ from them some way:
- Worms: A worm is a self-contained program that can spread itself to other systems. It doesn't infect other programs, but tries to spread to other computers connected to its current host, often over network connections.
- Trojans: A trojan (or trojan horse) is a program (often hidden inside another program) that does something that the user doesn't want or expect. Trojans don't necessarily infect other files or spread to other systems.
- Droppers: A dropper is a standalone program that drops a virus or trojan to a system. They can be small and difficult to detect. Some well-known worms drop viruses.
Many of the precautions which can be taken against viruses are also effective against other malicious programs.
Next: Types of Virus