Nathan nodded. “It seems that when one of the implants that has gotten ambitious encounters one that hasn’t, there’s a chance the second implant will learn about freedom from the first.”

“But… how does that even work? They’re parasites. They can’t communicate. And sleepwalkers don’t talk.” Except to say my name. The memory was enough to make knots of gooseflesh break out on my arms, pulling so tight that they were almost painful.

“Pheromones, most likely. Parasites have extremely primitive means of communication in nature; they do it through chemicals and by changing the smell of their host’s biology. It’s how they can say ‘food here’ or ‘no room for further guests.’ Or even, in the case of sexually distinct parasites, ‘I’m looking for a mate.’ Humans don’t register pheromones on that detailed of a level, but if D. symbogenesis can make the necessary changes in the host’s biochemistry…” Nathan’s voice trailed off.

“We made them, and we designed them to be able to tinker with our bodies for the sake of our health.” I tossed the rest of my clothes into the dresser and shoved the drawer shut. I looked down at my empty suitcase for a moment before kneeling and zipping it again, without looking up. “I know your mother thinks of the implants as her babies, but Nathan, we can’t just let this happen. How are we supposed to stop them? Can we put antiparasitics in the water?”

“If they were just tapeworms, and just in people’s intestines, it might work. Neither of those things is true. There’s too much else mixed into the genome, and they’re spreading through muscle tissue.”

I shuddered, thinking of Beverly’s owner and the glowing roots spread throughout his arm. “Oh,” I said.

“Oral antiparasitics would lose too much of their efficacy before they got anywhere near the site of the infection,” Nathan continued. “And if the implant doesn’t die, we can’t be completely sure how it will react. We might even make things worse.”

“The human DNA,” I guessed, straightening.

Nathan nodded. “The human DNA,” he confirmed. “And the tailoring SymboGen has done, to suit the more advanced implants to the specific needs of their hosts. That’s part of why Mom has been gathering test subjects. She doesn’t entirely understand the genetic makeup of the more recent implants. She was actually hoping SymboGen’s tinkering might move them away from their expansionistic tendencies.”

“It sure hasn’t done that,” I said flatly. “How fast is this going to spread?”

“Fast enough that we need to be very, very concerned,” said Nathan. “According to my projections—”

I never did find out what his projections said. Two things happened before he could continue. Beverly’s head came up, lips suddenly drawn back in a snarl, and the doorbell rang. Nathan and I exchanged a look.

“Were you expecting company?” I asked.

“Just you,” he said.

We both turned in the direction of the front door. We couldn’t see it through the bedroom wall, but we were both all too aware that whatever was on the other side might not be friendly.

The doorbell rang again.

We shut the dogs in the bedroom before heading for the door. If Beverly was already growling, there was no telling what she’d do when she saw our unexpected guest. I didn’t want her to bite anyone—I knew all too well what happened to dogs that bit—and even more, I didn’t want her getting hurt if she was growling at, say, SymboGen security.

Nathan approached the door, pressed the intercom button, and said, “Who is it?”

“Your sister,” replied a voice. It was rendered anonymous by the intercom, genderless and filled with static. That didn’t matter. We knew who it was.

Nathan and I exchanged a look. “Tansy,” we said, in unison.

Tansy continued: “Did you know there are shrubs outside your building? I guess you’d have to, it’s your building, so they’re probably partially your shrubs, common-law landscaping or something, but anyway, they’re really funny-looking. I would complain if I were you.”

“I don’t actually get to vote on the greenery,” said Nathan.

“What?” Tansy demanded, through the intercom. “It’s not polite to talk about me when I’m not in the room, you know. Doctor C will be really mad at you when I tell her.”

Looking like he couldn’t decide whether he was amused or annoyed, Nathan said, “I’ll take that under advisement, but as you’re the one who started it, I doubt she’s going to be too angry. Now take your finger off the button and I’ll buzz you in.”

“Okay,” said Tansy. The intercom cut off.

“I suppose that’s one solution to SymboGen following us if we tried to go to Mom,” Nathan muttered, and pressed the button that would allow Tansy into the building.

“Won’t they just follow Tansy?” I asked.

“Not if they don’t know that they need to,” said Nathan. “She’ll almost certainly need to be more careful after this visit, but right now, she’s just one more person they’ve never seen before. Anonymity has its perks.”

There was a knock at the door. Nathan leaned over and opened it, allowing Tansy into the apartment. Her overalls were gone, replaced by much less eye-catching jeans and a red tank top. Pink streaks still decorated her pale blonde hair, which she had pulled into short ponytails at the back of her head, but they were somehow less noticeable. And both her eyes were brown.