Droppers and bombs are other virus-like programs. They wait on the system for a specific event, then launch themselves and infect the system. File infectors, also called parasitic viruses, are the most common. They attach themselves to executable files. Characteristically, these infect by overwriting part or all of a file.
Macro viruses make use of the fact that many programs have programming languages built-in.
The languages help users automate tasks through the creation of small programs called macros. Microsoft Office provides many of its own built-in macros and this type of virus became know because of its infection of Microsoft Word. When a document or template containing the macro virus is opened, the virus runs and does its damage. Viruses come in many different forms and some are more destructive than others. Some viruses may damage files or perform other forms of destruction.