We do that for it when we e-mail those infected files to other people or put them on a floppy for someone. A Trojan Horse program is not really a virus. It is a program that is often designed to cause damage. It may be a fun looking program that instead of playing a cartoon deletes your drive. It does not make a copy of itself and spread.
Worms are similar to viruses.
They do copy themselves, but it is from computer to computer. They require very little human help with this. Instead of trying to take over a computer and infect all the files like a virus, a worm's intention is to infect many computers. It may do it similar to the more famous viruses by sending itself to other computers through the e-mail client on the infected computer. Unfortunately, Outlook Express is notorious for being a victem of this scheme.