This is by no means a game-breaking bug, but rather a grammatical error or missing word. A quality issue nonetheless.

For the slim chance that one of the game developers is going to read this, here is the error description:

The glossary says

Disease:

A temporary sickness that debilitates the hero in some way. Can be cured by some combat skills or Camping skills, but usually needs treated in town at the Sanitarium.


This should either be

[...] but usually needs to be treated in town [...]

or


[...] but usually needs treatment in town [...]