Author Notes: Thank you to munchkinofdoom for the beta!

Charlie stares at the equations covering the boards, but they're blurring before his eyes. He can't see a solution. He can't see what the numbers are trying to tell him. The math isn't working anymore, and he can't seem to shut off the clenching feeling around his heart whenever he thinks of Amita lying in that stark white hospital bed, her face blotchy and her eyes pleading with him to help her. He clenches his fist around a piece of chalk until it crumbles into pieces. Slowly, he opens his hand and watches as chalk dust floats down to the ground. It's the fifth piece of chalk he's destroyed in the last half hour, but it isn't helping, because whenever he tries to concentrate, whenever he tries to focus on the math, all he sees is Amita.

She's dying slowly and there's nothing he can do.

"You have to find something," Don tells him, his eyes burning bright. There's something very like panic in those eyes and Charlie doesn't want to think about how dire the situation is if this is how Don looks.

"Don't you think I know that?" Charlie can feel the panic bubbling up inside of him and the urge to run to the garage - to close the door and never come out again - is almost overwhelming. There are math problems in there that call out to him. Theoretical problems that won't ever mean the difference between life and death. Problems that are simply chalk on a blackboard: nothing more, nothing less.

Don's hand is heavy on his shoulder. "I know you know that," Don says quietly. "But Amita isn't the only person. The latest figures show that..." There's a catch in Don's voice that Charlie's only heard several times before. "The latest figures show that a third of LA's infected."

The chalk dust seems to choke the air, a filmy cloud that wraps itself around Charlie's throat until he can't breathe and the world turns hazy before his eyes. Charlie looks around rapidly, not knowing what he's looking for. The boards that he's set up in the living room blur as he moves his head, the equations dancing before his eyes.

"Charlie?" Don's voice sounds urgent. "Do you have something for us? Anything? We need to trace this. Find out who released the agent."

The agent. It all sounds so clean and sterile to Charlie. It's nothing like what the disease actually does with its short incubation period and the way it eats away at all the major organs until complete shutdown. That's what's going to happen to Amita unless Charlie does something, but he can't seem to focus. This is something he's done hundreds of times before. Trace an agent to its origin point, but this time it's different. This time it's personal and with the short incubation period and massive numbers that have crippled their city, tracing it to its origin point is a monster of a task. He's been trying for almost twenty-four hours, ever since they realised what this was, and he's gotten nowhere.

"The president's declared a national emergency and all air travel out of the US has been grounded indefinitely," the tiny voice from the television announces as Charlie grabs a marker from the coffee table. He knows the problem. At least he hopes he has the problem. The boards are too small. He needs something bigger to write on.

"Charlie?"

But Charlie ignores Don as he runs up the stairs, his marker clutched tightly in his fingers. The bedroom walls are a pale cream colour and he knows that they'll be perfect covered with equations. His mind drifts to the image of Amita again, hooked up to the machines, under an isolation tent. Why hadn't he been there with her? Why didn't he go out to get the pizza instead of sending her? They ended up drawing straws for it, and Amita laughingly said that she always ended up with the short straw.

That was the night before last, when everything had seemed normal. They spent half the evening arguing over how many clothes they ought to take to England and, more importantly, how many books. And then everything changed and Charlie got the phone call that Amita had collapsed outside of the pizza parlour. He should be there right now. He should be there sitting beside her, holding her hand, but Charlie knows he can't. He can't save her by holding her hand, but he can save her with chalk and markers and numbers.

Charlie takes a deep breath. He knows where Amita went that night. He can work from there and from the sketchy data given to him by the FBI. He can save LA. The CDC is permanently connected to his laptop downstairs with experts working around the clock for a cure. Their words just seem to slide over Charlie's head. They talk about something that had has the structure of a hantavirus but with completely different symptoms and that seems to spread like the common cold.

"You can do this," Don says from the bedroom door.

Charlie nods tersely as he begins to write on the walls, blue ink bleeding into the plasterboard. He needs to do this. He needs to do this to save Amita. "Leave me alone," he manages to get out and his voice sounds strange, even to his own ears.

"Okay," Don says, "but if you need anything. Anything at all..."

But Charlie's already blocked out the sound of his voice. There's nothing but the swirling numbers in front of his eyes and the knowledge that he needs to do this. For Amita. Charlie coughs and wipes his hands free of chalk dust. He looks at the wall and writes, the numbers flowing out of him and the pieces of the puzzle coming together before his eyes.

There's another tickle at the back of his throat but Charlie ignores it. He can't afford to look away now, not when he's finally made a breakthrough, not when he has a chance to save Amita and everybody else. This isn't the end. It can't be. He'll solve the problem and then he'll run down do the hospital and hold her hand while the FBI finds the bastards who did this and then they'll cure her.

Charlie scribbles out an incorrect number and focuses his mind on Amita and being in England with her. He wipes his nose with his hand and ignores it when his hand comes back stained with blood.

He can't let her down.

Not now. Not ever.