A virus is a small program written to spread itself through your system. Often times the virus will replicate and spread to other disks so that it might infect more systems. While the virus itself may not be sneaky, often times the author of the program is sneaky. In the course of reproduction and infection most viruses adversely affect computer operation or data integrity. While some of the most exotic viruses cannot be removed from your computer without excising the data that the virus has infected, many can be removed simply by repairing that data.
Viruses can take many forms, some benign, others more devious and destructive.
It is possible for a virus to delete files, format disks, or make your system run slowly. When a computer becomes infected by one or more viruses, the outward symptoms do not always indicate the source. Viruses themselves are not crafty, intelligent, or sly. 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.