virus 
A self-replicating piece of malicious computer code that is embedded in a seemingly harmless program or file. Once the program is run, or the file is opened, the code "infects" the host computer, begins propagating itself, and delivers its payload--which can include destroying data and/or critical system files, sending out large amounts of e-mail, or inserting random text into documents, for example. Viruses often propagate via programs or files attached e-mail messages. Examples include Melissa in 1999, the Love Bug in 2000, and Klez in 2003.