When a document or template containing the macro virus is opened in the target application, the virus runs, does its damage and copies itself into other documents. Continual use of the program results in the spread of the virus. These viruses usually replace or attach themselves to COM and EXE files. Many non-resident viruses simply infect one or more files whenever an infected file runs. The process by which a virus makes a copy of itself in order to carry out subsequent infections.
Replication is one of the major criteria separating viruses from other computer programs.
These viruses attempt to conceal their presence from anti-virus software. Other viruses hide the actual size of an infected file and display the size of the file before infection. Stealth viruses must be running to exhibit their stealth qualities. Trojan Horse A program that pretends to have a set of useful or desirable features, but actually contains a damaging payload. VBS programs can be embedded in HTML files and provide active content via the Internet.