Written by a student at the Clausthal University of Technology in the REXX scripting language, it drew a crude Christmas tree as text graphics, then sent itself to each entry in the target's email contacts file.
In this way it spread onto the European Academic Research Network (EARN), BITNET, and IBM's worldwide VNET.
The core mechanism of the ILOVEYOU worm of 2000 was essentially the same as Christmas Tree, although it ran on PCs rather than mainframes, was spread over a different network, and was scripted using VBScript rather than REXX.
Some users would read the REXX code first and see comments at the top telling them it is a fun Christmas card for them to run.
The actual executable part of the worm was contained in several overly long lines (more than 80 characters) that were not visible unless the user scrolled the screen to the right.