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A program can be referred to as a virus if it meets at least five of the following criteria:
1. The ability to get information
In general, a virus requires a list of the names of the files in a directory. What for? So that he can obtain a list of files that can he contagious. For example, a macro virus will infect all the data files in MS Word, will find a list of files ending in * .doc. This is where the ability to gather the information necessary so that the virus can create a list / data all the files, and then sort them by searching for files that could be infected. Usually this data is created when a file is infected / infected with a virus or file virus program itself was opened by the user. The virus will soon be collecting data and put it (usually) in RAM, so if the computer is turned off all the lost data. But these data will be created again each time the virus was activated. Usually this data is stored as well as a hidden file by the virus .
2. The ability to check out a file
A virus must also be able to check out a file to be infected, for example, he served infect program extension * .doc, then he should check if the document file has been infected or not, because if it had, it would be useless to infect them again. It is very useful to improve the ability of a virus in terms of infecting the speed of a file / program. Commonly done by the virus is to have / mark the file / program that has infected so it is easy to recognize by the virus. Examples of labeling is such as providing a unique byte in every file that has been infected.
3.
The ability to replicate and infect themselves
If this is indeed a virus "bang-get", that is, without the ability there is a virus. The core of the virus is the ability hold itself by infecting other files. A virus has found a potential victim when he will recognize it by check. If you have not been infected with the virus will start its action transmission by writing the identifier byte in the file, and so on copy / write object code of the virus on the target file. Some of the common ways the virus to infect / reproduce itself are:
Infected files to be deleted or renamed. Then created a file containing a virus program itself using the original file name.
Virus programs that have been executed / load into memory will directly infect other files by means of riding all existing files.