There are several different types of viruses:
- File infector viruses: these infect program files, such as applications, games or utilities. They are often memory-resident, meaning that once they have been executed they remain active in the computer’s memory and can infect more programs. Examples include Jerusalem and Cascade.
- Boot sector viruses: hard disks store a small program known as the boot record which is run when the computer starts up. Boot sector viruses attach themselves to this program and execute when the computer tries to start up from an infected disk. Infected machines will often refuse to start. Examples include Michelangelo and Stoned.
- Multi-partite or polypartite viruses: these infect both boot records and program files and are very difficult to repair as the virus code must be removed from both locations. Examples include Anthrax and Tequila.
- Macro viruses: these infect data files, such as Word documents or Excel spreadsheets, rather than programs. They are very common and can be difficult and expensive to repair. Macro viruses are written using macro programming languages, originally designed to allow users to automate tasks within an application. They are easy to produce, so there are now thousands in circulation. Examples include W97M.Melissa and WM.NiceDay.
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