A rot worm, w, is a monster in SLASH'EM. It is very weak, but its bite can make you ill.
Although not normally randomly generated, spellcasting monsters can create them via summon nasties, meaning they are a reasonably common sight in the middle to late game. They also appear in the Wyrm Caves and the Lawful Quest.
Rot worms are capable of dealing delayed instadeath to any character without sickness resistance, and it is often quite easy to overlook the "deathly sick" message amidst all the other battle spam. Thus, if a demilich summons one, be sure to use your unicorn horn after the fight, as sickness has no further messages, unlike stoning, and it is far too easy to overlook the "Ill" indicator on the status bar.