“No, dear,” said Dr. Cale, almost gently. “I used a fish tapeworm. They used Taenia solium, the pork tapeworm. I was able to keep very little of their research once I came on board, because they’d been using an inherently dangerous parasite. Not that any parasites are completely safe—the one we eventually went with had its dangers even before we started tinkering with the building blocks of its DNA. I think you’ll find that very little about the official history of D. symbogenesis is actually true. Most of it is pretty fictions and lies that can’t be disproven this late in the game. The Intestinal Bodyguard worked the way that it was supposed to, Steven Banks became a hero, and the rest of the development team became… expendable. We were a liability. Richard was consumed with guilt over what we’d done, and I was considered too likely to talk. By that point, you see, Steven had created this whole backstory for us. We met in college, we were the best of friends, and so on, and so on. So if he did anything to hurt my family, I could discredit him by revealing his lies. It was the nuclear option, for both of us. We quietly agreed that I would fade into the background.” She pursed her lips, looking unhappy. “Not that I intended for it to be quite such a long absence, but life does have its way of throwing you little curveballs when you let yourself get too cocky.”

“Is this leading into why you’re in the wheelchair now?” asked Nathan.

Dr. Cale smiled. “I was wondering if you’d noticed that I was suddenly shorter. Then again, you’re a lot taller. Maybe you just thought this was what happened to all mothers as their sons grew up.”

“I’m a doctor, Mom,” Nathan said. “I understand human anatomy.”

“And have I told you yet how proud I am of you?” Dr. Cale rolled herself closer to the wall, pressing a button on the base of the light box. The nearest piece of wall began to glow a steady white, backlighting the four X-ray films displayed there. All four showed a human spine, from different angles—front, back, left side, and right side. I thought it might be the same spine, although I didn’t know enough about anatomy to be sure.

In all four images, a white mass obscured the lower part of the spine, just above the pelvis. It looked like someone had taken correction fluid and scribbled on the negative, wiping away large parts of the spine.

Nathan frowned, stepping closer to the light box. “Is this a tumor?” he asked, indicating the white mass.

“Not quite,” said Dr. Cale. She sighed. “Human testing was a priority at SymboGen. We weren’t supposed to test on ourselves, naturally, and I didn’t. I might be fond of cutting corners, but I wasn’t a fan of risking my own life when I had volunteers perfectly willing to risk theirs.”

“Are you telling me this is a tapeworm?”

“Let me get there, Nathan. I know you. If I don’t give you the background now, you’ll go racing off and never give me the chance to explain. I need to take things at my own pace. Can you let me do that?”

Nathan frowned at her, the light flashing off his glasses obscuring his eyes so completely that I couldn’t tell whether or not he was annoyed. I answered for both of us, saying, “We can be patient.”

“That remains to be seen.” Dr. Cale settled back in her chair, folding her hands in her lap. “The Intestinal Bodyguard went through several generations, with a wide variety of different genetic makeups. The generation that eventually went on the market was less… robust… than some of the early worms had been, and that was good, because those early worms had a tendency to grow more than they were supposed to. Because they grew so fast, they demonstrated the potential dangers of the Intestinal Bodyguard—primarily, that the tapeworm could endanger the host if it reached a certain size. The growth of D. symbogenesis was retarded to guarantee that it would reach that size only after more than two years had passed, creating a ‘safety margin’ where balance could be maintained between parasite and host.”

“That’s why the two-year replacement requirement,” said Nathan. “That never made sense to me. Nature doesn’t work on such a tidy schedule.”

“Exactly. They don’t die after two years. They never did. The antiparasitic drugs take care of the old tapeworm, and a new one is put in place without anyone realizing that there was ever a risk.” Dr. Cale shook her head. “If it had ever gotten out, it would have been the end of SymboGen.”

“And a lot of people would have been hurt,” I said.

Dr. Cale seemed to wave my concerns away, continuing, “Everyone was meant to forget about the early generations, even though the final product was heavily influenced by their design. When Steven sent the word that the IPO was coming, I saw the writing on the wall. Eventually, there’d be so much money in the picture that everyone who had been there in the early stages would be in danger. We’d know too much. Well, I knew I’d need some kind of insurance if I was going to guarantee my safety—and by extension, yours, Nathan. Since I didn’t participate in the human testing, I was a suitable host.”

The Intestinal Bodyguard was fiercely territorial, and wouldn’t tolerate the presence of another worm in the body. Supposedly, this meant that a second tapeworm introduced into the body would just fail to thrive, and would eventually starve to death. According to some of the stories Nathan had told me, the first tapeworm would actually attack and devour the second. I wasn’t clear on how that happened, since tapeworms weren’t supposed to be that intelligent. I was pretty sure I didn’t want to know.