By definition, a virus program is able to replicate itself. This means that the virus multiplies on a computer by making copies of itself. In most cases, if a file that contains virus is executed or copied onto another computer, then that computer will also be "infected" by the same virus.
A virus program contains instructions to initiate some sort of "event" that affects the infected computer. Each virus has an unique event associated with it. These events and their effects can range from harmless to devastating. For examples:
- An annoying message appearing on the computer screen.
- Reduced memory or disk space.
- Modification of data.
- Files overwritten or damaged.
- Hard drive erased.
The different types of viruses are as follows-
1) Boot Sector Virus :-
The number one position on a hard drive is the Master Boot Record (MBR). The MBR contains the partition Table—the data specifying the type of hard disk and the partitioning information.The terms “Master Boot Record”, “partition table”, and “partition boot record” are
Often used interchangeably, although they are not exactly the same thing. Some Programs, such as the Boot sector Viruses, always attack the physical first sector: the Master Boot Record on hard disks.
Examples of boot- sector viruses are Michelangelo and Stoned.
2) File or Program Viruses :-
Some files/programs, when executed, load the virus in the memory and perform predefined functions to infect the system. File-infecting viruses generally (though not exclusively) attach to a file in one of
three ways. Some file viruses prep end, or bind themselves, to the beginning of the file, so that they run first. Other files append, or connect themselves to the end of the file, but modify the beginning of the file so that the virus runs first. Others overwrite some part of the existing file They infect program files with extensions like .EXE, .COM, .BIN, .DRV and .SYS .
Some common file viruses are Sunday, Cascade.
3) Multiplicative Viruses :-
A multiplicative virus is a computer virus that infects multiple different target platforms, and remains recursively infective in each target. It attempts to attack both the boot sector and the executable, or programs, files at the same time. When the virus attaches to the boot sector, it will in turn affect the system’s files, and when the virus attaches to the files, it will in turn infect the boot sector.
This type of virus can re-infect a system over and over again if all parts of the virus are not eradicated.
Ghost ball was the first multiplicative virus, discovered by Fridrik Skulason in October 1989.
Other examples are Invader, Flip, etc.
4) Stealth Viruses :-
This term has become so popularly debased as to include virtually any virus that neither asks permission to infect nor announces its presence by a characteristic message, graphic,sound effect, and so on. These viruses are stealthy in nature means it uses various methods for hiding themselves to avoid detection. They sometimes remove themselves from the memory temporarily to avoid detection by antivirus.
5) Macro Viruses
This is a new type of virus that use an application's own macro programming feature to distribute themselves. Unlike other viruses, macro viruses do not infect programs; they infect documents.
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