“Don’t you?” asked Dr. Cale.

Nathan looked away.

“The fact that you don’t want to answer that tells me that you’ve already learned one of the unpleasant secrets of D. symbogenesis, even if you’re trying not to admit it to yourself. Don’t worry; you will. You’re going to have to,” said Dr. Cale grimly. “The implants aren’t just a mix of human and tapeworm DNA. Again, that would never have worked. Oh, fooling laymen with science is sometimes so easy it should be criminal. How could anything be a chimera of two creatures and still be viable? You’d need something to connect the two. Something to blend them.”

“There’s nothing in the world that’s so malleable it could connect those two genomes.” Nathan looked back to his mother, eyes narrowed and angry. “There’s a point where it stops being science and becomes wishful thinking.”

“Toxoplasma gondii,” said Dr. Cale.

Nathan’s irritation faded, replaced first by horror, and then by an expression of sheer disbelief. “You’re telling me you spliced Toxoplasma into the genome?”

“Among other things, but it’s the Toxoplasma we need to worry about right now.” Dr. Cale beckoned for us to follow as she turned her wheelchair and began making her way toward one of the workstations. Adam walked beside her chair, while Nathan and I followed her. Tansy stayed behind to turn off the light boxes set into the wall. It seemed strangely responsible for someone so flippant, until I realized that letting the rest of us go ahead would give her access to our backs. I felt a lot less comfortable after that.

Then again, I hadn’t really felt comfortable since this whole thing began. Maybe a little more discomfort wasn’t such a big deal. I kept a firm grip on Nathan’s elbow, and followed Dr. Cale to the workstation.

The workstation had clearly been designed with accessibility in mind: the path to it was wider than usual, and there was more space below the desk, allowing her to pull her wheelchair all the way into place. Three computer monitors were arranged in a loose half circle, each of them displaying a screensaver of abstract loops and whorls of color twining endlessly around one another. Dr. Cale put her hand on the mouse, saying, “Scientists have known for years that the Toxoplasma parasite was capable of modifying the human mind in surprising and seemingly impossible ways. It still took us a long time to come around to that way of thinking. We didn’t realize Toxoplasma was capable of causing symptoms that mimicked schizophrenia, for example, until someone proved it.”

Nathan glanced at me. Apparently interpreting my expression as confusion, he said, “Toxoplasma is a common feline parasite. A lot of cat owners have it. Some people think that may be where the crazy cat lady stereotype comes from.”

“I know,” I said. “I work in an animal shelter, remember? I had to attend a hygiene class where we learned all about toxoplasmosis and how to avoid it.” Once a toxoplasmosis infection set in, it was virtually impossible to get rid of. The Toxoplasma parasite preferentially colonized the human brain, and most infections were mild enough that the cure was considered worse than the disease. Any antiparasitics strong enough to address the infection in the brain would wreck the host’s immune system, as well as killing off any more helpful parasites that might be in residence. It was an unnecessary risk. So we all wore gloves when we cleaned the cat boxes, and we were all careful around new cats, and things continued. But if the SymboGen implants contained Toxoplasma DNA, that changed everything.

I just wasn’t sure exactly how.

“I’m glad we’re all on the same page, then.” Dr. Cole opened a series of pictures, one on each monitor. One showed a tapeworm, curled in a large receptacle, as if prepared for dissection. Adam paled and looked away.

“Your original specimen?” I guessed.

“The portion that was removed from my body during the surgery,” confirmed Dr. Cale. She turned enough to pat Adam’s arm reassuringly. “That was long after the portion that would become Adam had been removed.” I got the feeling she added her last line as part of an ongoing argument, one where Adam blamed himself for her injury, and she tried, over and over again, to make him understand that it could never have been his fault.

The second screen showed a petri dish at thirty times magnification. It held a scattering of small parasites. Nathan frowned, leaning a little closer to the workstation. Dr. Cale leaned to the side, letting him get a clear view.

“The morphology is wrong,” he said. “They should be shorter and squatter, with no defined separation between segments.”

“This generation of Toxoplasma gondii had already been combined with some of the more desirable genes from the other creatures that would be contributing to the development cycle,” said Dr. Cale. “By this point, it was beginning to achieve a greater size, and seemed less interested in entering the brain, which was, you can imagine, rather important to us. Imagine the havoc a fully grown tapeworm could cause by attempting to migrate through the human body.”

“Havoc like seizures?” I asked very quietly. “Or like losing motor control and seeming to go to sleep while you’re still awake?”

“Havoc a great deal like that,” said Dr. Cale. The third monitor showed a blue crab for some reason. She tapped a key on the keyboard at the center of the desk. The image of the crab began to move, performing an odd stirring gesture in the water with its large front claws. It bobbed up and down as it stirred, looking content, if a crustacean can ever be said to experience contentment. “This was our last major contributor.”