“The common thought was that cowpox, the vaccinia virus, and the variola virus, which is the causative agent of modern-day smallpox, were all derived from the same ancient smallpox virus. But the Russian sample is just different enough that the cowpox and vaccinia aren’t a match. We tried the monkeypox virus, but that didn’t work, and then we discovered a horsepox from a breed of Mongolian horses with the right genome sequencing that was 99.7 percent similar to the ancient pox. Unfortunately, the horsepox we need is quite rare and we ran out of samples. We found a supplier of the more common form of horsepox and were able to synthesize a replica using a fragment strand, genome sequencing, and a DNA synthesizer. We then ran trials and discovered that it did create the right T cells and the memory B cells that can fight Smallpox R. R, for Russia. We call the virus S-poxR.”

“I’d call it Ivan. Or maybe Igor.”

“Are you kidding?”

He lifted a shoulder. “Maybe.” He smiled, a cute dimple forming on his cheek.

“Our problem now is that our only sample of S-poxR was destroyed in the fire. The CDC has smallpox in small quantities, but that is newer smallpox, samples that were taken from the 1940s to 1977, when the last case occurred. We need an ancient sample.”

“How are you going to get it?”

“The world got lucky. Zoey is actually a molecular anthropologist and has colleagues at the Mütter Museum of the College of Physicians of Philadelphia. They found in their Civil War collection what amounted to vaccination kits—old, person-to-person kits that doctors used to vaccinate soldiers against smallpox.”

“Person to person?”

“Crazy, I know, but the process was to take the fluid from smallpox pustules from one person and scrape it into the arm of the next person—just enough that they’d create antibodies. They also developed pustules and the procedure would continue.”

“That’s chilling. And gross.”

“Desperate times.”

“Right.”

“The CDC doesn’t have the right equipment to sequence these ancient DNAs, so the smallpox samples got shipped to the McMaster Ancient DNA Center in Canada, where they mapped the DNA. McMaster was working with small sections, maybe fifty base pairs long. But they were able to create an approximation to the ancient smallpox. And it was close enough that when it was injected into a subject vaccinated with the horsepox-based vaccinia, it was appropriately repelled.”

“Subjects?”

“Mice, monkeys, dogs.”

“Not humans?”

“That’s the next phase of trials, but because of the similarity to the current vaccinia, we have strong indicators that it will be effective, if necessary.”

Tae paused as the plane touched down. “Honestly, before we got our sample from Russia, I’d never really studied smallpox. Now, it scares me to death with the mortality rate and how easily it can be synthesized.”

“And you can re-create this Mongolian horsepox?”