Ms. Wu, who had been pretending not to listen to us, sat down on the other side of the couch. “I’ve treated fourteen cases of this so far this week,” she said. “At least, the symptoms and the onset of them has been the same. After the first six or so, I was driven to do a little research. The early symptoms include a dry mouth and nose, a painful, unproductive cough, a sensation of chill in the extremities, painful joints, headache, and fever. All of my patients have been Truebloods.”

The Ticker snorted.

“I beg your pardon?” Ms. Wu asked.

“I have no objection to being called an elf. I don’t believe it has any derogatory connotations.”

“Others among your people do.”

“My people,” Tick-Tick said, with fine dignity and a shot of a glance at me, “are widely assorted genetically, and are by no means limited to those who have their origin across the Border.”

I remembered my recent objection to her use of “my people” and “your people,” and grinned.

“Shut up, round ears,” she said, without looking at me.

Ms. Wu’s lips twitched, but she only said, “May I go on?”

“Please do,” I replied.

“I don’t know how familiar you are with the nature of viruses. They’re one of the most easily mutated organisms in nature, which is one of the things that makes them so hard to fight, in the World. By the time you’ve found something that will kill one, it’s likely to have turned into something else.

“True—elves living in the Borderlands have had very little to fear from viral infection, for the same reason. The Wall between the Elflands and the Borderlands seems to have a disruptive influence on any virus that passes through it, killing it or mutating it into something harmless. In particular, airborne viruses from the Elflands are unknown in Bordertown.”

I thought I recognized a prologue when I heard one. “Until now?”

“Until now. This seems to be a version of a disease known only in the Elflands, and not known well there. There, it’s not only rare, but slow to spread. No one I’ve talked to has figured out yet how it crossed the Border and became viable here. There may be a specific vector, which we can eliminate to slow down the spread of the disease.”

“Vector?” I asked.

“I’m sorry. Carrier, more or less. Rats were a vector for the Black Plague, in the Middle Ages.”

“Huh. Do rats cross the Border?”

“Only,” Tick-Tick supplied, “If they’re Trueblood rats.”

“So how do you cure it?” I asked, hoping to keep Tick-Tick from making any more outrageous comments.

Ms. Wu pressed the fingertips of her two hands together. “I don’t know,” she said.

The Ticker and I both stared at her.