“The crab?” asked Nathan. “Mother. Mom. I’m willing to believe that you combined two species of parasite and injected them with human DNA, but my willingness to ignore the laws of nature only extends so far. There’s no way you introduced crustacean DNA into the mix.”

“I didn’t. The crab isn’t a member of our donor species. This is a male blue crab infected by Sacculina carcini.”

“Same problem,” said Nathan, with the sort of dismissiveness I normally only saw him direct at orderlies who didn’t want to listen during his rare ER shifts. He didn’t want to hear what she was telling him. “Sacculina is a barnacle. It’s still a crustacean, and I don’t care if you’re a scientific genius, Mom. You’re not God.”

I guess having a lifetime of memories telling you how the world works is a lot more difficult to get past than six years of often-conflicting explanations. “Why can we combine parasitic worms and humans, but not parasitic worms, humans, and crustaceans?” I asked.

“Biology is tricky, Sal,” said Dr. Cale. “A lot of the rules are more like suggestions, or can be, if you come at them from the right angle, but you still want to break as few of them as possible. Break too many, and the chances that everything will go catastrophically wrong increase at an exponential rate.”

“We don’t count as things going catastrophically wrong,” said Tansy brightly, as she popped out of the darkness behind us. I jumped. Nathan didn’t, but from the way he tensed, it was a near thing. Tansy beamed. “We’re a natural evolutionary modification to an artificially created organism.”

“As I was saying,” said Dr. Cale. “Sacculina carcini is a crustacean, but it’s also one of the most dramatic examples of parasitic castration found in anything larger than a cone snail. It literally takes over and rewrites its host, turning a perfectly healthy crab into an incubator for the parasite’s own egg. One of the more interesting tricks in the parasitic castrator’s repertoire is the feminization of its host. You see, male blue crabs are aggressors. They’re likely to go out and get themselves hurt before the Sacculina babies can properly mature. That does the parasite no good at all—and neither does the production of sperm, which simply routes nutrients away from the Sacculina. So the parasite fixes all that by controlling the blue crab’s biology. It’s a very small creature, very primitive, and it still has the skill to turn a male crab into a female one, at least externally.”

“But you didn’t use it,” I said.

“No—we couldn’t, nice as that would have been. Barnacles simply weren’t compatible with the work that we’d already done. We would have needed to start over with something purely crustacean, and that would have made the human interface infinitely more difficult. Mr. Blue Crab here is simply intended to make a point.”

“And what’s that?” asked Nathan sharply.

“That parasites can control behavior on a much deeper and more integrated level than most people want to give credit to.” She tapped the keyboard again. The waving blue crab was replaced by an image of a simple flatworm. It was almost see-through, displayed in the classic backlit simplicity of a parasitology manual. “Meet Trichobilharzia ocellata, a member of a large, diverse family of trematode worms. They’re parasitic castrators, just like Sacculina carcini, although they’re biologically much closer to tapeworms. Much, much closer, after a little careful modification by yours truly.” Her smile held pride and regret in equal measure. “I’m very good at what I do. I always have been.”

Nathan stared at her like he couldn’t believe what he was hearing. “You mixed Toxoplasma and a parasitic castrator into the genetic makeup of the SymboGen implant?”

“Don’t make it sound like it was something accidental, Nathan, or something I did entirely on my own,” Dr. Cale said. She frowned at her son. “Every step I took was approved by the rest of my research team. Even Richard agreed that this was the only way we were going to get the implants to work—he wasn’t sure he wanted them to work, mind you, but he knew this was what we’d have to do to make them work. I never did find out what Steven had on him, to get him to join the team. I have to think it was even worse than what he had on me, because Richard was miserable. More than any of us, he saw how badly this could go. He understood in a way that Steven didn’t, and I…”

“You what?” asked Nathan.

“I didn’t want to. I had already given up my family for this project. I wanted it to work. I wanted to make scientific history, so that when we were finally able to have this conversation—which, I admit, went a little bit differently in my head”—she looked down at her wheelchair and grimaced before looking back to Nathan—“I wanted to be able to show you that I had made a difference. That it was worth it. I went out alone, I found the broken door, and I came back with all the riches we could ever have imagined. Things just didn’t work out quite the way I’d imagined them. That’s all.”

“Tell that to the dead,” said Nathan.

“I still don’t understand,” I said, interrupting before things could get even worse. I was afraid Tansy might do something if Nathan started yelling at Dr. Cale. I wasn’t clear on what “something” would be, but I couldn’t imagine it would be anything either of us would like. “What do all these other parasites have to do with the sleeping sickness?”