Computer virus - is a specially written small program that can "impute" themselves to other programs (ie "infect" them), as well as perform various undesirable actions on the computer. The program, which is located inside the virus, called a "victim". When this program starts, the first administration gets the virus. The virus is and "infect" other programs, as well as any adverse action (for example, the ports files or file allocation table on the disc, "choke" RAM, etc.). To mask the virus action to contaminate other programs and harm may not always met, and, say, if certain conditions. Once the virus is needed to fulfill his actions, it passes control of the program, in which it is located, and she also works as usual. In doing so, apparently the work of an infected program is the same as uncontaminated.
Many types of viruses so arranged that when you run an infected program the virus remains a resident, that is before DOS, in memory of the computer and occasionally infects programs and performs harmful actions on the computer.
Computer virus may spoil, that is to change improperly, any file on a disk in your computer. But some types of files virus can "infect". This means that the virus could "infiltrate" in these files, ie to modify them so that they contain the virus, which in some circumstances can begin its work.
It should be noted that the applications and documents, data files without the data tables Games and other similar files can not be infected with the virus, it can only spoil them.