I want to say "I guess they're not idiots" but the previous pages suggest otherwise.

Before the translation, I interpreted all of Reimu's lines as 'Are you an idiot?' or 'Are you stupid?' since pronouns are often implicit in Japanese.

Easy to make that mistake but if you re-read them all carefully, it actually makes more sense for Reimu to be referring to herself and not the girls.

And here I was waiting for the "idiots don't catch colds" payoff.

Now that I think about it, this probably is an "idiots don't catch colds" joke.
She's literally saying "not idiot".
In the first three, she's talking about herself, but in this one she probably means that they aren't, since they managed to catch a cold.
I changed the translation a little to reflect this.

Do you think "Isn't that dumb" would be a good translation. Other than the idiots don't catch cold joke, it's also meant that they're dumb enough to caught Reimu's cold.

bakudan00 said:
Do you think "Isn't that dumb" would be a good translation. Other than the idiots don't catch cold joke, it's also meant that they're dumb enough to caught Reimu's cold.

"You're not idiots" would still work in that case, as an implied "you should be smarter than this."

I've updated the translations a bit.

Marisa is human, so it's normal if she get sick!
Aya is a tengu, so maybe its normal for her to sick.
Remilia is a vampire, so its...wait,vampire get sick!???

BrotherJohn said:
Marisa is human, so it's normal if she get sick!
Aya is a tengu, so maybe its normal for her to sick.
Remilia is a vampire, so its...wait,vampire get sick!???

I don't even know if Tengu get sick...

Marisa is normal flu.
Aya is bird flu.
Remilia is bat flu wait what is that?

alivealive said:
Remilia is bat flu wait what is that?

http://en.wikipedia.org/wiki/White_nose_syndrome ?

Wait, idiots don't get colds right?

NWSiaCB said:

http://en.wikipedia.org/wiki/White_nose_syndrome ?

No sir, that's due to a fungi, not a virus. Also, it makes bats hyper... which flu obviously does not.

Indefinity said:

Wait, idiots don't get colds right?

On every single page of this sequence, including this one, Reimu says the exact same thing. "Baka janai no" - the literal translation is simply "Not idiot". It's common in Japanese to use as few words as possible, and let context fill in the rest, so it'd be easily understood as "I'm no idiot" on the previous pages.

However, it should be noted that if you use rising intonation on the "no" particle, it becomes a (feminine) question. If you don't, it becomes a (feminine) forceful statement. The intonation is not defined, so there are a bunch of different ways to translate this. Yes, one of them is "You're not idiots." Another one is "Aren't you idiots?"

However you want to take it, there's no one right answer. The ambiguity of the Japanese language is the intended vector of the joke.