Rocky held his sister tight. He didn’t sob, but tears spilled down his cheeks. Alex put her arms around him from behind, holding him even as he held JoJo. My face burned as I related details of Hank’s death—I knew as well as anyone that a child should never have to know too much about that—but I also realized that everything was different now.

We couldn’t lose track of our emotions, certainly, but we couldn’t give in to them the way we used to. Maybe Rocky and JoJo would need this information someday. Dr. Chatterjee certainly needed it now.

I finished telling him about the scene at the water tower and said, “Like those ants in that video you showed us. With the parasite?”

He took off his glasses and polished the lenses on his rumpled button-down shirt, though they did not look in need of polishing. “Ophiocordyceps unilateralis,” he said quietly. “The pieces are starting to fit together.”

“How?” Patrick said.

“Those adults out there”—Chatterjee pointed a trembling finger through the doorway of the nearest classroom to the windows and beyond—“have been infected.” He shook the detector, the words blinking out at us again: UNIDENTIFIED PARTICULATE. “This parasite attacked their white matter.”

“So why didn’t it attack ours?” Alex asked.

“You’re teenagers,” he said. “You have less.”

Patrick drew back his head. “We do?”

“Of course. Kids have a lesser-developed frontal cortex.”

“Rub it in,” Alex said.

“Look,” Chatterjee said. His hands shaped the air as they did when he was in teacher mode. He seemed to forget that one of them was gripping a clawhammer. “Every year from childhood on, white matter wraps around more and more of the nerve cells of the brain—that process is called myelination.”

“What is white matter?” Rocky asked.

As the sun inched up, squares of light from the windows stretched across the classroom floor opposite us. Some of the male Hosts had drawn closer to the school, spiraling their way around the front parking lot. One man in a scuffed denim jacket drew closer to the fence, his shoulder rat-a-tat-tatting along the chain-link, the sound sending electricity up my spine. Dr. Chatterjee took Rocky by the arm, drawing him out of sight past the doorway, the rest of us following. Sensing that something was wrong, Cassius leaned into my leg, his black-mask face pointed up, no doubt reading the stress coming off me.

“White matter transmits information from different parts of the body back to the cerebral cortex,” Chatterjee said, the hammer wagging by his face. “Which helps with executive function—decision making, attention, planning, motivation. Think of the myelination of axons as creating information pathways, connections that allow communication between all the parts of the brain. That’s what maturity is, really. Teenagers grow more white matter every year. But the last part of the brain to be myelinated is the frontal lobe.”