I mean, not everyone can cure the spanish flu for example, as it required the development of vaccine and so extensive medical knowledge that isn't generally present in the medieval-like worlds and/or that the "typically-normal everyone person" that reincarnates had prior to that.
Except for Isekai Yakyoku. The Black Death is present also but it is not the only one and certainly not the first thing the MC encounters and cures.
Ah, but it's true that Scorbut is also a good candidate, and it's quite "popular" too, as you can meet it quite often also in the series that calls for a pandemic.