@raltzero Could be either. (Por que)(Porque)(Por qué)(Porqué) all have different meanings/usages, but people use them interchangeably (they shouldn't)
For example, in @justforthelulz 's meme the correct term should be '¿Por qué no los dos' (why not both?); in your case, the closest to a 'Because?' would be '¿Porque sí? (assuming Because? is an affirmation question)
Those mind-altering Japanese über-colds is just one of those tropes you have to live with, no matter how silly they seem to us. Or maybe it's less of a cold and more of a flu and it's just one of those things were they don't really have two words for it?
@raidensnakeezio
Just to be sure, I checked with a native Japanese person, and they disagreed with your point. っつってんだろ doesn't make it sound very pleading. If anything, っつってんだろ (= って言ってるだろ) in general sounds pretty strong, like "I said that, didn't I? [So why aren't you listening?]"
And to begin with, it's not like I used an exclamation point, so the translated version doesn't even sound that strong.
That is to say, I see no need to change that line.