A computer virus is a program or piece of code that is loaded onto a computer and is capable of attaching itself to other files and replicating itself repeatedly, usually without the user's knowledge or permission.
A virus can be transmitted through an attachment to an email, by downloading infected programming from other websites, or through a floppy disk or CD. Some viruses will activate as soon as the infected file is opened, while others will lie dormant in the computer system until activated by a trigger. The trigger could be reaching a specific date or activating a particular function (such as reading an email). While some viruses replicate themselves without causing any further damage, most will also attempt to carry out other activities (known as the virus 'payload'). This can range from sending random emails to deleting the entire contents of your PC.
Other virus-related attacks include worms and trojans.
- Worms differ from regular viruses in that they only reside in computer memory, rather than attaching themselves to system files. Many email viruses (such as Melissa and the Love Bug) are worms.
- Trojans are pieces of code hidden in what appears to be legitimate software.
For instance, a game you download from the Internet may also contain code to monitor which Internet sites you visit.
The best protection against computer viruses is to use anti-virus software. By being proactive and keeping such systems up to date, you can significantly reduce the risk of your business being harmed by viruses. Anti-virus software should be installed on all your business and home PCs. Once you are a licensed user regular automatic updates will be sent to your computer from the vendor site when new viruses emerge.
Other steps you can take to protect against virus attacks include:
- being cautious about opening unsolicited emails, especially if they contain attachments
- only download software from trusted sites
- disconnecting your PC from the Internet when not in use.
Further information
Protecting - Where do I start? - What are the sources of threats to my information systems? - Viruses 
