A computer virus is simply a computer program. A Virus program is written by a malicious individual to spread itself by saving copies of itself on any computer disk it comes in contact with.
Some computer viruses do nothing but replicate themselves.
The only problems are caused by the space these programs occupy. The less important part in virus terms, but pretty important in computer-health terms, is what the virus program does after that. There are actually some virus programs that don't do anything other than part one. However, some mean individuals have created programs that delete all the files on a computer's hard drive, or subtly change the code files of all the programs on the computer so they will begin to crash. You are running a computer program called an operating system, which controls WHAT the computer does.