How Viruses Infect

Viruses are sometimes also categorized by how they infect. These categorizations often overlap the categories above and may even be included in the description (e.g., polymorphic file virus). These categories include:

• Polymorphic Viruses
  Viruses that change their characteristics as they infect.
• Stealth Viruses
  Viruses that try to actively hide themselves from anti-virus 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.
• Spacefiller (Cavity) 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.