Computer viruses were named such because of their similarities to the biological virus process. A virus program will attach itself to good or healthy files on the computer system and proceed to spread and infect other files on the system. Eventually, through email, open ports or network shares, the infected files make their way to new hosts and begin the process again.
There are various areas of the computer that can be infected by a virus or malicious code and there are various methods defined for how the infection occurs. The information below is from the Computer Knowledge Virus Tutorial (cknow.com). There may be other lists or terms, but these listings of what and how viruses infect are fairly comprehensive:
What Viruses Infect
- System Sector Viruses: These infect control information on the disk itself.
File Viruses: These infect program (COM and EXE) files.
Macro Viruses: These infect files you might think of as data files. But, because they contain macro programs they can be infected.
Companion Viruses: A special type that adds files that run first to your disk.
Cluster Viruses: A special type that infects through the disk directory.
Batch File Viruses: These use text batch files to infect.
Source Code Viruses: These add code to actual program source code.
Visual Basic Worms: These worms use the Visual Basic language to control the computer and perform tasks.
How Viruses Infect
- Polymorphic Viruses: Viruses that change their characteristics as they infect.
Stealth Viruses: Viruses that try to actively hide themselves from antivirus or system software.
Fast and Slow Infectors: Viruses that infect in a particular way to try to avoid specific anti-virus software.
Sparse Infectors: Viruses that don't infect very often.
Armored Viruses: Viruses that are programmed to make disassembly difficult.
Multipartite Viruses: Viruses that may fall into more than one of the top classes.
Cavity (Spacefiller) Viruses: Viruses that attempt to maintain a constant file size when infecting.
Tunneling Viruses: Viruses that try to "tunnel" under anti-virus software while infecting.
Camouflage Viruses: Viruses that attempted to appear as a benign program to scanners.
NTFS ADS Viruses: Viruses that ride on the alternate data streams in the NT File System.