How do worms and viruses spread?

From a list I'm on. The constant spread of a worm is being discussed.

these pc's were set up by the help desk staff and i know they just use blank admin passwords when setting up a pc because these accounts are never used and if a help desk admin has to boot a pc into safe mode, they don't have to remeber 500 diff passwords.

You get what you ask for. Sometimes without saying a word...