Computer viruses are a self-replicating segment of a computer code that resides in a host program. To explain how a virus works, look at how a computer operates. A computer executes instructions, which consist of machine code. A virus is a code that someone has written and placed inside the computer. Viruses basically perform two simple functions: they copy themselves to other programs and then they perform the task that the creator has written. The writers who created these viruses have complete control over them. First, they decide what the virus will do. This consists of deciding how it will replicate and what type of damage the virus will do. Second, the writer has control over when the virus will perform the destruction. The writer can make it perform as soon as the program executes or they can delay it until a certain date or time.