A virusvirus: ultramicroscopic non-cellular organism that replicates itself inside the cells of living hosts is a simple program [program: a list of instructions written in a programming language ] designed to cause harm to someone else's computer. A virus spreads by duplicating and attaching itself to other files. The extent of the damage depends on the virus. There are tens of thousands of viruses.
Some virus damage is insignificant but inconvenient. Some virus damage is disastrous, putting the computer system out of action by erasing files or corrupting data [data: information without context, eg a list of students with numbers beside their names is data, when it's made clear that those numbers represent their placing in a 100 metre race, the data becomes information ].
Viruses are written by malicious programmers who wish to cause problems for other computer users.
The primary source of infection these days are email [email: electronic mail - a message written or typed on a computer and sent electronically rather than by post ] attachments [attachment: a file that's 'attached' to an email is called an attachment ] followed by illegal software [software: a general term used to describe an application or a program ] and infected files from the Internet [Internet: a global network connecting millions of computers ]. If you have up to date anti-virus [anti-virus: anti-virus software scans all forms of storage devices for viruses and, if found, attempts to remove them ] software installed this will immediately warn you of any infection. If not, there is usually no evidence of the virus and the user is not usually aware of it until something goes wrong.