More fever dreams and mixing with my day job reading about biochemistry. Why no necromancy fireball? I GOT YA NECROMANCY FIREBALL!
Casting Time: 1 action
Range: 100 feet
Components: V, S, M (a lymph node cut from a plague victim)
Duration: Concentration, up to one minute
You fling the dried lymph glad to a point in range and it explodes in a black mass of smoke. Each creature in a 20-foot-radius sphere centered on that point must make a Constitution saving throw. A target instantly becomes infected with a pox if it fails the save becoming covered with boils and pustules. Any creatures who make their save are unaffected. Each turn an affected creature takes 3d6 necrotic damage at the start of its turn while affected. Any ability that removes diseases ends the effect on an affected creature. Creatures immune to disease are immune to this spell.
At Higher Levels: When you cast this spell using a spell slot of 4th level or higher, the damage increases by 1d6 for each slot level above 3rd.