When the file is loaded into memory, the virus can infect still other files, spreading itself even further.
Viruses often have harmful side effects.
A Trojan horse, much like its mythological equivalent, is malicious computer program that disguises itself as something harmless. When the program is run, however, the malicious functions are performed. The destructive effects may or may not be noticed by the user. Trojan horses, unlike viruses, are incapable of self-replicating. Melissa is a recently discovered Microsoft Word for Windows 97 macro virus.