A computer virus is a special kind of computer program which: Spreads across disks and networks by making copies of itself, usually surreptitiously. Can produce undesired side-effects in computers in which it is active. How infection occurs In order to infect a computer, a virus has to have the chance to execute its code. Viruses usually ensure that this happens by behaving like a parasite, i.e. by modifying another item so that the virus code is executed when the legitimate item is run or opened. Good vehicles for viruses include the parts of a disk which contain code executed whenever that disk is booted, and documents which contain macros executed whenever that document is opened with the relevant application. As long as the virus is active on the computer, it can copy itself to other files or disks that are accessed. Virus side-effects As well as self-replicating code, a virus normally contains a 'payload'. The former is like the propulsion unit of a missile; the latter is like the warhead it delivers. The payload can be programmed to have malicious side-effects. These effects can range from harmless messages to data corruption or destruction. How viruses spread Infections spread from machine to machine, and from organisation to organisation, in a number of ways. Viruses can be transmitted by: Booting a PC from an infected medium. Executing an infected program. Opening an infected file. Common routes for virus infiltration include: Floppy disks or other media that users can exchange. Email attachments. Pirated software. Shareware.