D&D has never really had a great use of disease.
The mechanical role of disease is as a way to inflict a condition that robs characters of the option of rest (rests don't restore you, they make you worse), and drain your resources over long periods of time (so, when between adventures, or while travelling over land for many days).
But, a lot of D&D diseases get forgotten about because they don't directly impact the fight or the dungeon crawl the party is currently having. Diseases can last between sessions (because they impact between adventures), and so get lost. And it's relatively easy to remove them for a well-prepared party (a paladin or a cleric does fine).
There's space, though. My group had a run-in with mummy rot that's still ongoing. Kind of the exception that proves the rule, though.
Makes me want to make a DMs Guild supplement for diseases that can actually be used in play and impact the party.