Dr. Chatterjee blinked a few times. “Well,” he said, “I suppose I don’t.”

“No,” Chet said, still rocking himself. “You’re right.”

“How do you know?” Chatterjee asked. “Chet? How do you know?”

“I … um, I saw my neighbor—Mr. Gaeta? Right after it happened. He was chasing a kid down the middle of our street, and a car…” Chet gasped a few times. “I saw his brains when they … spilled out. And they were black. Like covered with oil. And then next…” His breathing quickened, and for a moment I thought he might hyperventilate. “The car plowed into the kid he’d been chasing.” He took in a gulp of air. “Luis Millan.”

At this a wail went up from the back of the gym. Probably one of Luis’s cousins. We were all shocked.

“His head was…” Chet’s hand hovered by his forehead. “And his … brain … I could see … it looked normal. It wasn’t all black and oily. Not yet. So no, I don’t think the stuff was in there. I think it waits in the air until the second we turn eighteen. Your brain’s ready, and then that next breath costs you … everything.” He stared at his trembling hands. “Like Britney.”

Alex pulled the cuffs of her sweater down over her fists. She jackknifed over, her feet up on the bleacher bench in front of her, her arms pressed between her thighs and her chest. Patrick sat beside her, rubbing her back.

Again I looked across at my brother’s name and birthday written up on the board. Then down at the wet smudge from the mop where Britney had fallen.

I didn’t mean to speak, at least not that loudly, but there was my voice, carrying across the gym. “How could they know?”

“How do they know any of it?” Ben said. “They know to burn the guns. They know to cut the power. And the phone lines. The grown-ups—it’s like they’re still in there somewhere, but just the bad parts.”

Beside me, Alex shook off a shudder.

I said, “What I mean is, how could the parasite know exactly when Britney turned eighteen?”

Rocky said, “Well, Dr. C. said the white matter—”

“I know, I know,” I said. “But everyone develops at different rates. I mean, we’re humans. It’s not like we’re trees and you can just cut us open and count the rings inside. I know that doctors can make guesses based on teeth and bone development and stuff, but it’s not like we have some internal meter or something. Besides, nothing can tell when we actually enter the world. I mean, as opposed to conception or being in the womb or whatever.”

“If there is a meter of some kind,” Eve said, “maybe it starts the instant air first hits the lungs?”

“But there isn’t one.” I looked over at Dr. Chatterjee. “Isn’t that right?”