A computer virus is malicious software that spreads by attaching itself to files or creating files that may be executed in some way. Usually a computer virus is sent to users as an email attachment. A computer virus may require a computer software vulnerability to spread depending on the type of program the virus uses to spread. A computer virus runs on a system against the owner's or user's wishes and knowledge.

The computer virus may alter data and files on the infected computer. Computer viruses attacks at the application layer