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“Of course. What do you want to know?”

“Start with her medical history. Maybe there’s something unusual in her system that wasn’t in the other victims.”

Tim called up Walker’s records, scanned through the usual list of military checkups, inoculations, physicals … then found exactly what Margaret was looking for: something unusual.

“She had lupus,” he said.

Margaret shook her head. “That can’t be it. I can’t see how an autoimmune disease would affect the crawlers. They hijack stem cells to produce copies of themselves.”

Tim looked deeper in the record. When he found the next difference, he felt his heart start to hammer.

“Jesus, Margaret … Walker underwent HAC therapy to treat the lupus.”

Margaret narrowed her eyes, not understanding. “What’s HAC therapy?”

That question surprised Tim. She hadn’t just tuned out from life, she’d tuned out from medicine altogether. She wasn’t even reading research journals.

“HAC is human artificial chromosome treatment. It’s an experimental way to treat genetic defects. The process introduces a new chromosome into stem cells. The end result is stem cells with forty-seven chromosomes instead of the normal forty-six that all cells are supposed to have. The forty-seventh chromosome probably has a myriad of immune system modulators meant to reprogram cells to stop the autoimmune effects of lupus — new transcription factors, genetic code to modify gene response, et cetera. In some cases HAC even introduces fully artificial gene sequences.”

Even as he said the words, it struck him how similar the process sounded to the Orbital’s infection strategy: targeted changes to the host’s DNA, altering the processes created by millions of years of evolution. Was humanity that far away from harnessing the very technology that threatened to wipe it out forever?

Margaret’s eyes narrowed. Her nostrils flared. Normally, she looked like she couldn’t hurt a fly, but now her expression was that of a predator.

“An artificial chromosome in stem cells,” she said. “Maybe the Orbital’s technology can’t properly integrate that forty-seventh chromosome.”

She nodded, slowly at first, then gradually faster.

“This therapy,” she said. “Where did Walker get it?”

“Let me check.” Tim read through Walker’s records. “Looks like a clinic within the Spectrum Health System in Grand Rapids, Michigan. Cutting-edge stuff, only ten people in the trial.”

Margaret thought for a moment. Her excitement seemed to grow.