They are simply programs written by someone being malicious. These virus programs are written in such a way that the average user would not realize a virus had infected their floppy disk or data. This is why virus scanning software and virus cleaning software was created. Virus software is your first line of defense against trouble. A virus can live on any floppy disk, hard disk, or removable diskette media such as a Zip disk or Syquest.
Viruses live in the data on the disk.
Many viruses live in the boot data on a disk so that every time you use the disk to boot up, it can spread itself to any new disks you insert. Some viruses live only inside of programs, when you run the program they spring into life, and often their goal is to crash your computer or erase your data. These viruses are called trojan horses. Viruses are usually spread by infected floppy disks or infected programs which may be downloaded over a network, often as e-mail attachments. The original Trojan Horse, according to legend, was a huge, hollowed-out wooden horse given to the city of Troy as a gift from the Greeks.