Worms

The word “worm” or “tapeworm” was coined in the 60's when computers were run with the help of punched cards or tapes and user interaction was minimal. The extra code was thus introduced into the computers via computer tapes and hence it got the name tapeworm from some quarters. The worm used to copy itself onto other tapes and attack only data media. This may lead you to conclude, quite correctly; that the virus history dates back to the 60's. While this is true, it should also be borne in mind that such “worm” code was harmless and was used mainly to play pranks on co-workers.
The main difference between a worm and a virus is that a worm spreads by duplicating itself and does not need a carrier program (Trojan) while a virus needs a “host” program to spread. As such, worms are found more in multi-terminal (LAN) environments rather than on stand-alone PCs. In such systems, the worm replicates by moving from the memory of one machine to another.

The main difference between a virus and a Trojan is that a Trojan does not replicate but spreads only when it is copied. A Trojan is very similar to a Bomb except that it has a delay mechanism while a Bomb explodes at the first instance. For this reason, a Trojan is sometimes known as a “Time Bomb”.

Medium of Propagation

Viruses infect hard disks and floppies. In several countries, viruses have spread mainly through Bulletin Board Services (BBS), exchange of floppies or through LANs. Certain people believe that BBSs are solely to blame for the spread of viruses because of their “open architecture”, whereby anybody with a modem can access the system. We feel this is unfair. Watchful SYStem OPeratorS (SysOps) of many boards do take proper and ample care to keep their boards virus free.