The terminology is based on how the malware spreads. Worms actively exploit holes in network programs (e.g. emailing themselves to your contacts), trojans disgu

The terminology is based on how the malware spreads. Worms actively exploit holes in network programs (e.g. emailing themselves to your contacts), trojans disguise themselves as something useful (e.g. a pirated game). True viruses spread by injecting copies of themselves into innocent files in such a way that opening the file triggers the payload. True viruses are rare these days because the infection vector is passive and relatively slow: an infected file must be transferred by user action to another computer, for example by sharing an infected file via floppy disk.