Viruses
The classic definition of a virus is a piece of code that inserts itself into a file or an application. When the file is opened or the application is run, the virus relies upon the resources and capabilities of that application to insert itself into other files or applications in an attempt to propagate to other computers. Viruses usually, although not always, have an intent beyond copying themselves. These other purposes can include virtually anything that the application or other applications or utilities on the computer are capable of doing: opening a computer’s firewall and setting up share points to allow malicious users into the computer, spreading by email or by scanning a network for other vulnerable computer, capturing personal information, destroying files, using the computer as a tool to attack other computers on the Internet, or leaving the computer open to remote control by an unknown malicious user are among the nasty things that viruses routinely do to computers.