on behavior modification of mammals by single-cell parasites. Eric read it and tracked me down. Have you ever heard of Toxoplasma gondii?"
"I was an English major, when I was anything," I said. "If Shakespeare wrote a sonnet about it, I might have run into it. Otherwise, no."
"It's a really cool organism," he said. "Pretty much the classic example of parasitic mind control."
"Parasitic mind control?" I said. My flesh crawled a little.
"In mammals at least. There are some pretty great ones for insects and mollusks too, but if you want to play with hosts that have spinal cords, T. gondii is the best game in town."
Aubrey's eyes were bright, and he leaned forward over the steering wheel as he spoke. Enthusiasm made him seem younger than he was. I kind of wished he was getting jazzed about something with a lower ick factor, but as he went on, the urge to wash my hands lessened a little and I found myself getting interested.
"It usually lives in a cat's intestinal tract," he said. "We call the cat the final host. It's where the organism really wants to be."
"So what does it do to the cat's mind?" I asked.
"Nothing. Zip. Nada. But there's a middle part. In order to get from one cat to another, it passes through mice. So the first step is to go from the inside of a cat to the inside of a mouse."
"And you do that by...?" I asked just a heartbeat before I figured it out. I made a face. "We're about to talk about mice eating cat poop, aren't we?"
"Well, yeah," he said. I weighed whether to change the subject back to mystical assassins and my recent slaughter thereof, and reluctantly decided to stay with the poop-eating mice. We paused at a red light. Two homeless men passed beside the car, faces flushed with the heat.
"The thing that's interesting is what happens once it's inside the mouse," Aubrey continued. "Normally, mice avoid anyplace that smells like a cat's living there. Just good sense. But infect a mouse with T. gondii, and it isn't afraid anymore. In fact, it starts liking the smell. The infected mouse starts hanging out where cats are more likely to be. Good for the cat, because it's more likely to get a meal. Good for the parasite. It can get into a fresh host. Lousy for the mouse."
"Okay, that's the creepiest thing ever," I said. "I think I get it, though. That's like riders. The things that are inside Coin? And the ones we killed last night?"
The light turned green. Traffic started moving.
"Some riders can be like that, yeah," Aubrey said. "I don't think the Invisible College ones are quite that flavor. But there are also a lot of riders that will just hang out in the back of someone's mind and...change them. You know?"
"The way your amoeba thing changes mice," I said.
"Actually, the way it changes people. T. gondii infects humans too. People with the cysts in their brain suffer mild disinhibitions. Men become more prone to violence."
"And women?"
Aubrey glanced over at me and then back at the road.
"Sex," he said. "It makes women more affectionate and prone to...ah..."
"Get prone?" I suggested. A green sedan cut in front of us. Aubrey swore, hitting the brakes and his horn at the same time. I took the opportunity to switch subjects.
"So Eric read your paper and tracked you down?"
"Yeah," Aubrey said. He seemed relieved not to be talking about sex. I wasn't sure whether I was or not. "He was working on an idea about riders. See, there are some things about riders that look a lot like biological agents. And then there are things that really just don't. What we were doing was sort of reverse-engineering riders. Figuring out what kinds of constraints are on them from the way they act."
"Hey, that was Stout," I said, pointing back at the street sign we'd just passed.
"It's a one-way. They all are downtown. We'll go down Champa and turn around."
"Okay," I said. "Sorry. You were saying? Reverse engineering something?"
"Yeah, like cicadas. Did you know cicadas have prime-numbered cycles?"
"I did hear about that, yeah," I said. "Something about staying away from things that eat them, right?"
"That's the theory. If the cicadas are trying to avoid a predator with a five-year cycle, they develop a thirteen-year period and only coincide with the predator every sixty-five years."
"Okay," I said. I was getting a little lost, but I didn't want Aubrey to think I was stupid. "So what's