I'm not aware of a demon lord associated specifically with disease, and none of the archdevils fit that bill fully either. However there were several yugoloths (daemons in 1e) that were overtly linked to disease - in fact the whole disease motif was associated with them as a type of fiend, as an extension of their and the Gray Waste's style of blind, uncaring agony and spiritual decay.
Unique 'loths with some sort of disease association, or potential one:
Mydianchlarus (current Oinoloth)
Anthraxus (former Oinoloth, and altraloth)
Typhus (altraloth)
Bubonix (altraloth, Overlord of Carceri)
Cholerix (altraloth)
Xenghara (altraloth)
Taba (altraloth)
Daru ib Shamiq (baernaloth)
Harishek ap Thulkesh (baernaloth)