This doesn't require a grasp of anything especially complicated. Maybe I'll ask for some contaminated food to run tests on.

Maybe you're immune to uncleanliness because you're aliens, that'd be something.

I'm pretty sure we are. No one has gotten sick who didn't eat contaminated food? Not the doctors who treated them, not their spouses who kissed them or cared for them while they were sick?

I don't think anybody's eating the food who's married to someone who doesn't also have to - doctors have special procedures so they can treat people who have gross diseases without carrying the gross -

And it happens even if the food is subsequently washed and cooked and prepared?

Yeah. I don't know, maybe doctors have a way to do special food procedures too, but I think they'd tell people.

Probably. Do reds ever get this particular sickness?

Of course they don't, they're already always unclean.

Being unclean protects you from getting sick from contaminated food? Does being unclean have any bad effects?

It is a bad eff-

The kid is cut off suddenly.

He frowns.

The next day he asks their hosts if he can have some samples of the poisoned food to do tests on, and also whether they expect the poisoned food to have any effect on people who are already unclean.

"Oh, of course it wouldn't do anything to reds, we let them have it when we found out since it wouldn't make any difference to them - there's not many of them but what unpoisoned food we diverted from their usual supplies helped a little - how are you planning to handle it?"

"The highest standard of caution we have is for the cores we use for our power plants, and that's worked for centuries without incident; we'll use that until we figure out what's going on."

"And this doesn't involve you coming into direct contact with the tainted food, or with any unwashed instruments that touch it -?"

"There will be six inches of solid metal between the experimenters and the food."

"All right. I can have you brought some samples out of the red district - do you care what kind of food -"

"A variety would be good. We're trying to figure out if uncleanliness has detectable physical properties that could perhaps be fixed with technological intervention."

"I believe some of our scientists are already trying to come up with a way to purify it, but didn't you say you had a harvest goddess who was going to come fix everything?"

"Yes, I just find it intellectually curious."

"All right."

The samples arrive in several layers of packaging. Most of it looks like it'd be shelf-stable - some grain, some dried fruit - but they also seem to have stolen someone's sandwich; there's a bite out.

Perhaps the person didn't like their sandwich. He takes it back and runs tests. It's not radioactive. It does have bacteria, of course; they get to culturing and cataloguing them. "There are other, subtler things that could be wrong but it's odd that they wouldn't affect any reds -"

     "Maybe they've already been exposed?"

"For a virus that would explain it, for some sort of protein-folding foodborne disease not so much - but we don't know enough about local biology yet, maybe -"

Elves are dispatched with a few more sample requests. 

He inquires after local musical traditions.

Music is a green thing! Except for drumming, which some greys do too. He has lots of green-haired people happy to talk to him about music.

He is a happy Elf! - they're not especially talented but they're doing interesting things, that's something. 

Ten days after they arrive the courier lightleaper comes back to announce that Yavanna's coming - and Melkor wants to come too - but was in the middle of something and won't be here for another week. The ship is packed with food to tide the locals over in the meantime. It is also packed with really really pretty tapestries for the Elves to promptly drape all over everything; they do that. 

He didn't come the first time because he was feeling unreasonably averse to dying but he comes with this one.