Severus or Harry (or both) have an allergy. Litterally, that's it.

BONUS:

*How severe is the allergy? Is it Epipen level or is it more of an intolerance? (If you have both, are their allergies on the same level or different?)

**Include how they deal with the allergy (how does the magic world deal with it? Spells? Potions? Magical Epipen?)

***If Harry and Severus both have the same allergy.