Viruses can infect a number of different portions of the computer's operating and file system. These include:
System Sector Viruses infect executable code found in certain system areas on a disk. There are boot-sector viruses, which infect only the DOS boot sector, this kind of virus can prevent you from being able to boot your hard disk, and MBR viruses which infect the Master Boot Record on fixed disks and the DOS boot sector on diskettes. All common boot sector and MBR viruses are memory resident.
File Viruses infect applications. These viruses usually infect COM and/or EXE programs, though some can infect any program for which execution or interpretation is requested, such as SYS, OVL, OBJ, PRG, MNU and BAT files. File infectors can be either direct-action (non-resident) or resident. A direct-action virus selects one or more programs to infect each time a program infected by it is executed. A resident virus installs itself somewhere in memory (RAM) the first time an infected program is executed, and thereafter infects other programs when they are executed or when other conditions are fulfilled. Most viruses are resident.
Macro Virus a program or code segment written in the internal macro language of an application and attached to a document file (such as Word or Excel). Infect files you might think of as data files. But, because they contain macro programs they can be infected. When a document or template containing the macro virus is opened in the target application, the virus runs, does its damage and copies itself into other documents. Continual use of the program results in the spread of the virus. Some macros replicate, while others infect documents.
Companion Virus uses a feature of DOS that allows software programs with the same name, but with different extensions, to operate with different priorities. Instead of modifying an existing file, creates a new program which (unknown to the user) is executed instead of the intended program. On exit, the new program executes the original program so that things appear normal. Most companion viruses create a COM file which has a higher priority than an EXE file with the same name.
Cluster Virus modifies the directory table entries so the virus starts before any other program. The virus code only exists in one location, but running any program runs the virus as well. Because they modify the directory, cluster viruses may appear to infect every program on a disk.
Batch File Virus uses text batch files to infect.
Source Code Virus adds code to actual program source code.
Learn more...
| 
