Viruses today are typically spread via email or Internet browsing, but can also be spread by sharing USB drives, network connected drives, or Internet downloads; including downloads from Peer2Peer and Bit Torrent applications.
Occasionally, they are accidentally spread within packaged software products.
Viruses cannot spread themselves, but must be run (or executed) by someone to cause damage. For example: Boot sector viruses spread when a user inadvertently boots his or her workstation from an infected floppy disk. Macro viruses can spread by simply opening an infected document.