Viruses are programs that copy themselves throughout a computer or network. Viruses can only be activated when a user opens the program. At their worst, viruses can corrupt or delete data, use the user's email to spread, or erase everything on the hard disk. If not checked, a virus can damage the system or specific files which are important for the functioning of an operating system. It may also slow down our computer as a result of replication of virus in the background without being visible on the screen.