A virus is a self-replicating program that produces its own copy by attaching itself to another program, computer boot sector or document.
It infects other programs,
Corrupt files and Programs
Boot sector virus: Replaces itself with boot sector moving boot sector into another location on the hard disk
File overwriting or cavity Virus: Replaces the content of files with some other content leaving the file unusable
Crypter: Encrypts the contents of the file which causes the file unusable for the user
Polymorphic virus: The virus code mutates itself by keeping the algorithm intact.
Tunnelling Virus: These viruses trace the steps of interceptor programs that monitor operating system request so that they get into the BIOS and DOS to install themselves. To perform this activity they even tunnel under anti-virus software programs
Metamorphic virus: They rewrite themselves every time, reprogram themselves into a completely different code and back to normal vice versa
Macro Virus: Infects Microsoft products like WORD and EXCEL. They are usually written in the macro language visual basic language or VBA
Cluster Virus: Modifies the directory entries so it always directs the user to the virus code instead of the actual program
Stealth/ tunnelling virus: They intercept the anti-virus call to the operating system and give back uninfected version of the files requested for thereby evading anti-virus
Extension Virus: Hides the extension of the virus files, deceiving the unsuspecting user to download the files.
Metamorphic Virus: As with a polymorphic virus, a metamorphic virus mutates with every infection. The difference is that a metamorphic virus rewrites itself completely at each iteration, increasing the difficulty of detection. Metamorphic viruses may change their behaviour as well as their appearance.
Add-on Virus: Add-on viruses append their code to the host code without making any changes to the latter or relocate the host code to insert their own code at the beginning.