A computer virus is an executable program. Depend on the nature of a virus, it may cause damage of your hard disk contents, and/or interfere normal operation of your computer. By definition, a virus program is able to replicate itself. This means that the virus multiplies on a computer by making copies of itself. This replication is intentional; it is part of the virus program. In most cases, if a file that contains virus is executed or copied onto another computer, then that computer will also be "infected" by the same virus. A virus can be introduced to a computer system along with any software program. For Internet users, this threat can come from downloading files through FTP (file transfer protocol), or referencing email attachments.