The aid spell increases your current and maximum HP, but only for the duration of the spell.
However, the School of Necromancy wizard's 10th-level Inured to Undeath feature (PHB, p. 119) says:
your hit point maximum can't be reduced.
Does that mean a character with the Inured to Undeath feature can increase their maximum HP permanently by having aid cast on them, since the increase from aid can never fade away even after the spell stops being active?
If this is the case, could repeated applications of aid over multiple engagements ratchet their HP upward each time and increase their HP infinitely?