In the virus dictionary approach, when the anti-virus software examines a file, it refers to a dictionary of known viruses that the authors of the anti-virus software have identified. If a piece of code in the file matches any virus identified in the dictionary, then the anti-virus software can take one of the following actions:
a) attempt to repair the file by removing the virus itself from the file;
b) quarantine the file;
c) delete the infected file.

Dictionary based anti virus software typically examines files when the computer's operating system creates, opens, closes or e-mails them. In this way it can detect a known virus immediately upon receipt. Note too that a System Administrator can typically schedule the anti-virus software to examine all files on the user's hard disk on a regular basis.