Sanae would say: "You must not go by lore Touhou is unlogical and logic is overrated in Gensokyo Thus vampires can catch colds It is an unchanging fact"
Or its fanfiction and the author is ignoring source material to make the plot do what he whats.
It's par for the course.
Y-Yes!?Remilia is special to Sakuya. And Sakuya is also special to Remilia.That is an unchanging fact....I thought as much.Moreover, lately Remilia has caught a mild cold, isn't that the reason Sakuya is being overprotective of her?You must not be jealous of Remilia.