"I mean, hopefully! I am going to work on that right now because vaccinations against smallpox have not been useful since before I was born so I haven't got one! But it's not magic the usual way."

"Not magic? Does that mean anyone can do it, not just you?"

"Yes, and they already have," comments Hammond. "Dr. Edward Jennings proved his method could work just a few years ago... Do you mean to say that everyone in your entire country is vaccinated in your time?"

"Do you mean... Jenner? Edward Jenner? Yes, we get vaccinated against all sorts of things but not smallpox because we don't need it. You can only get smallpox if there is smallpox around. There isn't in my time because we were particularly thorough about vaccinating people against it in particular and it doesn't have animal reservoirs or anything else complicated like that."

He frowns. "Not around at all? How can that be, when it could spread back from other places in the world?"

"We have already noted that humans, too, can spread around the world, and that includes humans possessed of vaccines! Transportation is a lot quicker in the future."

"But you would need to vaccinate all of them," he says, half-laughing at the preposterousness of the idea.

"Not quite! If you release a dozen mice on an island, the island has mice now; if you release one mouse, and it's not pregnant, onto the same island, it will eventually die and leave a mouseless island. If you have one unvaccinated person whose entire town is vaccinated, that one person may just never get smallpox coming in, or, if they do come by it somehow, they will never give it to anyone around them. You do have to have extremely good coverage. Not all of the vaccinations were strictly consensual even if you don't count children as examples of not being consensual. But you do not actually have to get every single human on the planet, just enough, and in the right places. - before I discovered that magic was real and pivoted to that, I wanted to be an epidemiologist. I suppose I could meet Edward Jenner, that'd really be something, but we're too early for John Snow..."

"Hmm," says Hammond, "I see." He isn't a biologist, but as a diplomat, this sounds highly impractical to him. But clearly he's missing a lot of information about her world.

"Anyway this has not happened here yet so I'm gonna try to mimic the effect with magic right now."

They fly onward, searching for anyone who can give them directions to Cusco.

They see humans once or twice, and land to approach them, only to be preempted by dragons who swoop down and pick them up before they can even say hello.

That's weird behavior but she can't really make progress on figuring it out.

She can't actually draw her diagram in the air - it has to be more precise than wingbeats allow - but she can do math and slightly ugly derivations, in pencil that she affixes to her hand with a spare hair tie so she won't drop it. When they take breaks she makes progress on laying out the castable diagram.

After this happens a couple of times, Iskierka gets frustrated.

The dragons sometimes separate briefly to cover more ground during their flights. After one of these, Iskierka comes back with an old man in her talons and Granby berating her for picking him up without asking.

"It is not as if I speak his language to ask him!" she cries. "And I just knew he would be hurried off again if I did not grab him. We can just have Hammond ask him the way and put him down where we found him again."

Can they? Does this guy speak Spanish or possibly Portuguese and/or does Hammond know random non colonial South American languages?

Definitely not Spanish. He speaks some form of Quechua. Hammond has in fact made some efforts to learn Quechua, but it's been pretty hard without written references or access to native speakers, and this guy doesn't speak the exact same version they speak in Cusco either.

Hammond manages to figure out:

* The man's name is Taruca.

* The man believes that Iskierka owns him now. 

* He sort of knows where Cusco is. He grew up there.

"I certainly hope you are planning to clarify to this gentleman at the earliest opportunity that you do not own him," May says to Iskierka.

"Well--"

"YES, SHE IS," Granby says firmly. 

Grumble.

Hammond attempts to make it clear to Taruca that they just wanted directions and intend to return him to where he was as soon as possible. 

Taruca seems frustrated and upset by this. "Then why did you take me?!" he says. "Tupay," his previous dragon, "was fine, but--"

It turns out he was previously abducted by another dragon and removed to her ayllu, as they call it. He was not especially attached to his most recent dragon. In fact, he was first abducted as a child from Cusco, many miles from where they are now. He is old now, and pox-scarred; this was some fifty years ago.

"I am blind, but there is still useful work I can do," he says. "You don't have to return me to Tupay. Besides, she will be long gone after seeing such a large, powerful ayllu."

This is going to be all but entirely lost on May if Hammond is not attempting to translate for more general benefit than that of the officers because it involves a lot of mostly incomprehensible back and forth.