A computer virus is a type of Malware. It is a program or piece of code that is designed to damage your computer. It does this by corrupting your system files, wasting your computers resources (such as RAM, CPU time etc), changing or destroying your data or otherwise causing problems.
A virus is really contagious - but it does need humans to do something to help it spread.
Viruses can spread by:
- Email attachment - you open the attachment and the virus runs
- Instant Messengers - infected links
- File sharing sites - once an infected file is uploaded to a shared folder it gets downloaded to everyone who is synced with that folder
- Software downloads - you think it is software, it is a virus, or the virus is part of the download
- Unpatched software - which can be exploited to let viruses spread.
To stop viruses you need antivirus software (fairly obvious) but you also need to be careful that you don't click attachment and links that are not safe as antivirus only stops viruses it has seen before.
Pure computer viruses don't really exist much anymore though - as other more complex types of malware, often hybrids, are used instead. Some of the early viruses were fairly harmless, the Creeper virus just displayed the message "I'm the creeper, catch me if you can!". Some of the more recent ones (which were really hybrids) did a lot of damage, for example - the Sobig.F virus caused $37 billion in damages and stopped an Airline flying for for a while.
Do computer Viruses still exist?