Virus A computer virus is an unwanted program which runs on your computer and attaches itself to a host program on your disk. When the infected host program is run, the virus is activated and attempts to replicate itself and spread.
Many viruses lie hidden waiting for some trigger event before revealing their presence.
This trigger event can be different things such as after a virus has replicated a given number of times or on a specific date. Once activated, some are maliciously destructive while others simply display an annoying message, although even benign viruses can do unintentional damage while trying to multiply. You can't get the virus by just being online, by calling another computer or by reading a text email message. Currently, computer viruses are categorized by their mode of infection. This centers on the path used to replicate the virus and the type of file or program infected.