What is a computer virus?
The difference between a computer virus and other programs is that viruses are designed to self-replicate (that is to say, make copies of themselves). They usually self-replicate without the knowledge of the user. Viruses often contain 'payloads', actions that the virus carries out separately from replication. Payloads can vary from the annoying (for example, the WM97/Class-D virus, which repeatedly displays messages such as "I think 'username' is a big stupid jerk"), to the disastrous (for example, the CIH virus, which attempts to overwrite the Flash BIOS, which can cause irreparable damage to certain machines).
Viruses can be hidden in programs available on floppy disks or CDs, hidden in email attachments or in material downloaded from the web. If the virus has no obvious payload, a user without anti-virus software may not even be aware that a computer is infected.
A computer that has an active copy of a virus on its machine is considered infected. The way in which a virus becomes active depends on how the virus has been designed, e.g. macro viruses can become active if the user simply opens, closes or saves an infected document.
How infection occurs:
Once the virus is active on the computer, it can copy itself to (infect) other files or disks as they are accessed by the user. Different types of viruses infect computers in particular ways; the most widespread types are Macro, Boot and Parasitic viruses. Next, we will explain exactly what these are and how they operate.