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