The term virus is more recent, and was first used in 1984 by Professor Fred Cohen to describe self-replicating programs. The earliest PC viruses came a bit later, in 1986-7. The name is appropriate, because like a biological virus, a computer virus is small, makes copies of itself, and cannot exist without a host. All computer viruses at least take up disk space, and many of them are able to remain in the computer's memory, so as to take control over some computer functions. In addition, some viruses are poorly written, and may cause the computer to halt, or damage files.
Many viruses make the computer's memory unstable, or cause programs to run improperly.
Then there are viruses created in recent years that have been deliberately designed to destroy data on the disk. The most feared viruses are those that deliberately damage or delete files, or even destroy all data on a disk. The vandals who produce these are concerned only about themselves, not innocent people who will be harmed. These vandals hope to impress their friends, and sometimes compete with them. Some join groups that create new viruses every day.