A computer virus is a program that can make copies of itself.
Whether or not a specific virus does anything more depends on
the maliciousness and skill of the person who created the virus.
Some viruses "trigger" on certain days and might erase every file on
your hard disk. Others are more insidious, making small changes to files
that may go unnoticed for months. Once such a virus is discovered, it
may be too late to recover all modified files, as the alterations may
be in backup copies as well as the file currently in use.
