According to SearchSecurity.com there are three main classes of viruses:
File infectors
System or boot-record infectors, or
Macro viruses.
File Infectors
File infectors attach themselves to files, usually .COM or .EXE files.
The virus is loaded when the infected file is executed or ran and then
the virus is installed at the same time. Other times the virus is sent
through e-mail and the virus is sent as an attachment. When the attachment
is ran, the computer becomes infected.
System/boot-record infectors
System/boot-record infectors are viruses that infect system information
areas of a disk. These viruses are attached to the boot sector of a floppy
or infect the Master Boot Record on a hard drive. The usual mode of infection
is someone with a infected floppy accidently leaves it in the drive and
when the computer is rebooted the computer looks at the floppy first,
and then the virus is released. Many times this causes a temporary loss
in function for that computer.
Macro Viruses
Macro viruses are said to be the most common viruses, but they tend
to do the least amount of damage. According to SearchSecurity.com "Macro
viruses infect your Microsoft Word application and typically insert unwanted
words or phrases.
