A program (usually an executable program) that infects a computer file by inserting a copy of itself into the file. These copies are usually executed when the infected file is loaded into memory, allowing the virus to infect other files. A virus requires human involvement (usually unwittingly) to propagate. When a virus is active in a host computer, the infection can spread rapidly throughout a network to other systems.

Some viruses may be benign and result only in amusement or slight annoyance. Others can be malicious and destroy or alter data.