A computer virus is a self-replicating program which is written and distributed for the sole purpose of being copied onto a computer's hard disk without the user knowing about it, and to adversely affect the operation of that computer. Once activated, a virus spreads by replicating itself and attaching copies of itself to other files.
Some viruses can cause file damage.
Others may just display messages. Be sure to scan ALL floppy disks that you mount on your Mac, especially those from outside your immediate computing environment. You never know where those floppy disks have been, or what they've been exposed to. Don't forget to scan floppy disks from newly purchased, shrink-wrapped software packages too. While the probability of infection of such floppies is low, it's not completely unheard of.