A companion virus doesn't modify the infected program and usually passes control to the original .EXE file, but once detected it is easy to clean you simply delete the relevant .COM file. Cleaning disks infected with a link virus requires a specific approach. Every file virus can incorporate different techniques to improve the infection rate or to avoid detection. Each of the above viruses can be memory-resident, can have stealth capabilities, can use encryption or can use a polymorphic engine. Technically another form of parasitic virus, the thing that makes macro viruses rate a class of their own is that they are transmitted as an executable component in an otherwise non- executable data file.
Macros are executable code intended to automate tasks in applications.
However the underlying macro language is extremely powerful, and can call out to external programs, making macro viruses potentially quite dangerous. Macro viruses have rapidly become the most reported viruses in the world. This is a virus that infects both boot sectors and executable files and exhibits characteristics of both boot sector and parasitic viruses. This is a virus which hides its code or even a whole infected file by encrypting it. The only plain text that can be seen inside an infected file is a decrypting procedure.