2.1.1 What do we mean by 'infect'?
A virus will attach itself in various ways to a file that already exists on a 'host' computer, and when that file is run, the virus activates as well. A computer virus works in a similar way to a biological virus.
Biological virus: an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants or bacteria.
Computer virus: an infectious program of small size that can only multiply in other programs.
(Source: Encyclopaedia Britannica, 2000)
Several computers are shown. A virus, looking like a space invader, arrives at one computer and infects it, corrupting its screen. It replicates and the copies spread to the neighbouring computers and infect them.
When the virus is triggered it releases its payload. The payload part of the virus code can be either destructive or intrusive, or possibly both. Some viruses may just display a message, but others corrupt data and delete files.
Most viruses are programmed to hide on the host computer for a period of time before releasing the payload. If the virus shows itself too quickly, alerting the user to the fact that their computer is infected, it is more likely to be detected and hence less likely to be spread to other computers.