Platt’s heart pounded with every footstep. He felt the kick of adrenaline in his gut, a mixture of dread and anticipation countered the exhaustion.

The hallways were quiet, some dark. He avoided the elevators. Took the stairs instead. He needed the motion. He caught himself taking two steps at a time. Slow down, he told himself when he really wanted to break out in a run.

Dr. Drummond had told him that, “Dr. McCathy needs you on the fourth floor. He said you have to see this for yourself.”

Best-case scenario, McCathy was being his melodramatic self. Worst-case scenario, McCathy found something worthy of his melodrama, something to justify his pent-up anger.

Despite what he had told Agent O’Dell, Platt’s limited examination and observations of Ms. Kellerman had led him to draw some conclusions. She had been coughing up blood and had problems breathing, along with red eyes and obvious severe abdominal pain. Her fever had been high enough and had lasted enough days to cause fever blisters inside her mouth.

Her soiled bedding indicated bouts of vomiting and diarrhea that in the last twenty-four hours had rendered her so weak she hadn’t been able to get out of bed. She was in shock and remained unresponsive and incoherent. Early tests indicated that her kidneys had begun to shut down. If his preliminary assessment was correct, her other organs would soon follow.

Because of the severity of her symptoms he had narrowed the cause down to three possibilities, three biological weapons that a terrorist might use. None of them would be easy to treat. An anthrax infection, depending on what form, might be controlled with antibiotics. Hopefully they might be able to contain the spores to Ms. Kellerman’s house and to those already infected. Ricin would need minimal containment, as well. But if ingested, ricin was a deadly toxin and caused a painful death. The third possibility he didn’t like to even think about. If the terrorist had managed to use an infectious disease like typhoid or a virus like Marburg or—heaven forbid, Ebola, then treatment and containment might be impossible. Ms. Kellerman’s house would be a hot zone and anyone within reach of her or it could be a walking epidemic.

Platt slowed when he got to the fourth floor. The procedure would be for McCathy to prepare and seal his sample slides while in a space suit inside a Level 4 suite. Once preserved and sealed they would be able to look at the slides without fear of exposure. Platt knew he’d find McCathy now in the Level 3 suite where the electron microscope was kept. The expensive contraption was a metal tower as tall as Platt. It’s beam of light allowed them to see microscopic cells and view them like geographic landscapes.

Platt changed in the outer staging area from his jeans and sweatshirt to surgical scrubs, latex gloves, goggles, a paper mask and shoe covers. Then he joined McCathy.

The microbiologist sat at the counter, hunched over the binocular eyepiece of a microscope. When he looked up at Platt his eyes looked wild and enlarged. He wore thick eyeglasses under the goggles. His face and even his paper mask were damp with sweat. His neatly trimmed beard stuck out from behind the mask, giving him a crazy-scientist look that, ordinarily, Platt would have shrugged off as part of McCathy’s melodrama. This time it added to the thump already banging inside Platt’s chest.

“It’s not good,” McCathy said. “This is absolutely amazing. In fact, it’d be absolutely beautiful if it wasn’t so goddamn deadly.”

“What is it?”

“The cells from Ms. Kellerman. They’re busting open with worms.”

“Worms?” The banging in Platt’s chest invaded his head, as well. “Impossible. There must be a mistake.”

“Take a look for yourself,” McCathy told him, bolting up and sliding his stool aside, offering Platt a look through the microscope’s eyepiece.

Platt swallowed hard and moved in. Adjusted the focus. Tried to ignore his sweaty palms inside the latex gloves. He took a deep breath and clanked his goggles against the microscope’s eyepiece. What he saw looked like spaghetti or thin curlicue snakes with threads unraveling from their sides. They pushed against the cell wall, breaking away from a clump, or what they called a brick, in the center of the cell.

Platt forced himself to breathe slowly. Without moving, still staring, he said, “What about our own lab contamination?”

“Impossible. Our samples are in freezers, separated from this lab by three walls of biocontainment.”

“There are other things this could be.” But Platt couldn’t think of a single one. The cell had been invaded and was exploding with what looked like worms, snakes tangled in a pile. “This agent, this invader doesn’t loop much. And it’s too long. Shouldn’t there be a shepherd’s hook?”

“There’s only one thing I know of that looks like that, whether there’s a loop, curl or hook,” McCathy said. “I saw Marburg years ago. Samples taken from an outbreak along the Congo. Wiped out a whole village in a matter of weeks.”

Platt had seen something similar. The quarantine he had told Agent O’Dell about was one enforced from an outbreak of Lassa fever, another single RNA virus. But Lassa didn’t make cells explode like this.

“How can we confirm it? I don’t just mean running the cells through the electron microscope. I mean inexplicably. We have to be certain, without a doubt,” he told McCathy. They couldn’t waste any more time.

“We can test Ms. Kellerman’s cells against the real thing.”

“What do we have to do?”

“We take more of her blood serum and drop it on cells, on samples from our freezers, samples that we know have the real thing. If any of them glow…” McCathy shrugged. “Then you have your confirmation, beyond a doubt.”

“What do we have in the freezer?”

“Marburg, Ebola Zaire, Lassa and Ebola Reston.”

“How long will that take?”

“I can suit up now.” McCathy glanced at his wristwatch. “Take about thirty to forty minutes to prepare the samples from the freezer. Once I drop Ms. Kellerman’s cells onto the real deal it’s a matter of minutes.”

“Okay, let’s do it.”

“Wait a minute. I work alone in Level 4.”

Platt wasn’t surprised that McCathy would balk even at a time like this. He kept calm and steady. He didn’t raise his voice, didn’t allow a hint of anger when he said, “Not this time.”