What I'm wondering is whether allergies might register as phobias before people understood what allergies were. Like, in the story, our guy says "the presence of a cat in the same room with me makes me feel like nothing on Earth. I can't describe it, but I believe quite a lot of people are affected that way. Something to do with electricity, or so they tell me." Which to me seems like it could very plausibly be 'OH GOD I CAN'T BREATHE WHEN THE CAT'S AROUND, WHY????'

Also, why have I not read this Mary Stewart novel?!