What type of virus is most likely to remain undetected by antivirus software?

A cavity virus attempts to write itself into empty portions of a program file, filling 'cavities' without altering the file's length, but this doesn't inherently make it universally stealthier than a dedicated stealth virus.

A stealth virus employs techniques to actively hide its presence from antivirus software and the operating system. It may intercept requests for file information and provide clean, uninfected versions of files or hide changes to file sizes and boot records to avoid detection.

A file-extension virus (a less common classification) might attempt to deceive users by using misleading file extensions, but this doesn't prevent antivirus from detecting the malicious code itself.

A macro virus infects applications that use macros (like Microsoft Office documents) and is generally detectable by antivirus software designed to scan document files for malicious macros.