Virus
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A small piece of software that ¡°piggybacks¡± on another
program. This ¡°piggybacking¡± is really the virus copying its
reproduction code and payload to
said program. The reproduction code would be the section of the virus that finds, through some method, a target file to infect next. The payload of the virus is what the virus does other than reproduce. A payload can be anything ranging from a ¡°Hello world¡± pop-up, to the disk being erased. What triggers the payload is just that; the trigger. The trigger of a virus is whatever might cause the payload to happen. Usually this trigger is a specific date in the system clock, or a counter for the number of times the virus has reproduced. This trigger exists because if the virus¡¯ payload happened immediately, it would be very simple to counter the problem before it became a larger-scale issue.
Typically, the above is achieved by having the virus read the header of the host
file, these bytes are then saved into either the virus, or appended to the end
of the file. Now, in place of this old header is written a jump statement,
directing the program to wherever the virus¡¯ body is to written. Usually, this
would be the bottom of the file. However, there are other methods to this, and
more ambitious viruses will even split the virus body into separate sections
which take the place of any ¡°holes¡± in the host program.
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