Once infected, your computer may infect every diskette put in your floppy disk drive, as well as erasing or damaging files on your computer. You can very eaily be a source of infection without knowing it. As the name implies, a Trojan Horse program comes with a hidden surprise intended by the programmer but totally unexpected by the user. Unlike viruses, Trojan Horses don't make copies of themselves. Like viruses, they can cause significant damage to a computer.
Worms are like viruses in that they do replicate themselves.
However, instead of spreading from file to file, they spread from computer to computer, infecting an entire system. Worms most often cause damage by overwhelming computers and networks with thousands of copies of the worm. Although they are not truly viruses, a general discussion of computer viruses is often assumed to include Trojan Horses and Worms. Back up your data often, in several places if possible. Use an up to date virus scanning software package.