A virus is a computer program written by someone, with the presumed intent of spreading and causing grief. Like a human virus, a virus makes the infected computer “sick”: it causes poor performance, crashes, lost files and data, or more.
Also like a human virus, a computer virus replicates itself. Just as you can copy a file from one disk to another and have copies on both disks, a computer virus is in part defined by its ability to make copies of itself. Exactly how a virus does this depends on its type, but can include propagation over removable media such as USB drives, networks, or network-based activities such as user downloads.