A modified version of this public domain image.

Goblin Punch’s latest post introduces a monster that can make lights become “thin” and “greasy” and eventually die so that the party can be consumed by the darkness. What I love is that the thing affecting the torches and candles is a disease that lights can catch. This harkens back to his idea of rust being a disease that infects metal.

What other phenomenon from our world could be reimagined as a disease in our RPGs? This was originally a thread on Mastodon.

Knots

Ropes, hair, chain can contract a tendency to tie themselves in ever more complex knots when left alone. In the advanced stage, all such materials wriggle free of the backpack and start entangling everything nearby.

Lies

In addition to mundane problems of rot & mildew, the written word can contract virulent falsehoods that spread through a library.

The virus understands humans poorly, and mostly changes random details. Its only consistent effect: clumsily encourage the reader to consume fragments of the book, hinting this will grant transcendent power. Instead, it causes fever, hallucinations, and eventually vomiting up blank (infected) tomes.

Locks

Infected doors slowly grow locks, typically baroque in decoration. No key grows. If the door is opened the door will gradually shut and lock itself again.

The disease spread from door to door on your hands; it can only survive more than 24 hours on metal or wood (e.g. lockpicks).

Creatures that regularly feed it by pouring blood into the keyhole can win the door’s affection. It will then open for them, unless frightened.