A hand slapped the window next to Frankie and she jumped. A woman was running alongside the bus, her eyes wide and wild. Sweat dripped down her face, hanging in beads off her chin. Her mouth flapped open in a cough.

Beth gasped and clutched Frankie's arm.

Outside the bus the woman gasped, her chest heaving. Something yellow dribbled out of her mouth. Frankie turned away, sickened.

Something hit the bus on the other side and someone screamed. Frankie leaned across the aisle to look out of the opposite window. Someone else ran alongside the bus - she couldn't tell if it was male or female beneath the thick woollen cap it wore. The figure gasped, throwing its head back. Frankie got a quick impression of glazed eyes before blood and bile rushed out of the gaping mouth.

"What the hell . . .?" The driver's voice was a panicked shout.  

A man stood in the road ahead. His chin and shirt were stained red, sweat plastering his hair to his forehead. The driver honked the horn but the man didn't move, didn't give any indication he was aware a bus was bearing down on him. The driver swore and spun the wheel, swerving the bus to one side to avoid turning the staring man into a messy splatter across the road. A screech of brakes came from the other lane.

"What's going on?" Beth cried. Fear made her look about eight years old, trembling because the monster under the bed was showing its face.

More figures ran at the bus. Bloody hands slapped the windows, leaving red smears on the glass. People had giggled at Lesley Philips because they didn't know what else to do.  

No one was giggling now.

Frankie pulled Beth away from the windows, frightened that the people on the other side were going to smash through and pull them out onto the street.

Tyres screeched as the bus swerved to avoid another person standing in the road, and Melly fell out of her seat. She scrambled to her hands and knees, her face blanched with fear. Frankie grabbed her hand and pulled her up.

The driver muttered under his breath. Frankie couldn't hear what he was saying but there was no mistaking the fear in his voice. That chilled her almost as much as the bloody-mouthed figures chasing the bus. At fifteen she was well on her way to adulthood, but that didn't mean she'd surpassed the age where grown-ups were who you turned to when everything went wrong. They didn't get scared or confused, they always had the answers, and they always sorted out whatever was going wrong. Knowing the bus driver - the only adult on the bus - was as scared as the rest of them, felt like a smack in the face.

The bus picked up speed as it went over a bridge, and the figures started to fall back, unable to keep up.

"It's not flu," Beth whispered. Frightened tears shone in her eyes. "They've got whatever Lesley and that guy at lunch had."

A chill ran through Frankie. Beth was right. The people launching themselves at the bus had the same signs; the clammy skin and glazed eyes, the coughing. But their sickness seemed so much worse than what had happened at school. She looked at the bloody smears on the windows and her stomach roiled. Lesley had hacked up bile. These people were throwing up blood.

"Everyone okay back there?" the bus driver asked. His voice was shaky.

No one got a chance to answer. A woman ran into the road ahead, waving her arms over her head. "Stop," she screamed. "Please, stop."

The driver hit the brakes and the bus screamed to a halt. The impact threw Frankie back in her seat. Melly fell to the floor again.

The woman ran to the door and banged on it. "Let me in, please," she cried. Her voice was hoarse, breathless.

The driver hesitated. Frankie didn't blame him. After what they'd just seen she'd have been hesitant too. Except the voice sounded familiar. She climbed to her feet and approached the front of the bus.

Mrs Sanford stood outside. Her face was sweaty but it didn't have that clammy look the sick people did - more like it was sweaty because she'd been running. Her eyes found Frankie's and relief dawned there.

"Frances, thank God. Let me in, please."

Frankie looked back at the driver. "Open the door."

He didn't look happy but he complied. The door hissed open and Mrs Sanford clambered onboard. There were sweat-stains under her arms and her lipstick was smudged. She sank into the nearest seat and clenched her hands together to stop them trembling.

"It's everywhere," she whispered. She sounded . . . haunted.

Frankie crouched in front of her. "What's everywhere?"

With a lurch the bus started up again, and Frankie had to grab the edge of the seat to keep from falling. She noticed the grass-stains on Mrs Sanford's knees where she must have fallen at some point.

Mrs Sanford gestured wildly with her hands. "This . . . thing . . . this sickness. It's everywhere, all over town. I drove home and . . ." Her voice trailed away.

Beth whimpered. Melly hadn't even bothered to pick herself up off the floor this time. She sat in the aisle, her face pale.

Mrs Sanford sat up straighter, a jerky movement. "We have to get out of town, right now."

"What?" Frankie said. "We can't -"

Mrs Sanford grabbed Frankie's arm. Her hands were damp. "You don't understand. Those people out there" - she gestured to the blood smeared on the windows - "there's something wrong with them."

"No shit, Sherlock," Vanessa snapped from the back. Her voice was brittler than normal, fear sharpening her words.

Despite the situation, Frankie half-expected Mrs Sanford to turn around and chastise Vanessa for her attitude. But the teacher acted like she hadn't even heard. Her eyes remained locked on Frankie, her lips trembling.

"Whatever this is, it's happening all over town," she said. "I just saw my neighbour . . . she said  there were people vomiting blood in the aisles in Tesco."

Frankie prised Mrs Sanford's hand off her arm. "I don't understand."

She knew what she'd seen but part of her brain refused to accept it. It wanted to block out what had happened, to somehow strip away the fear and the uncertainty so she wouldn't have to deal with the possibility that something seriously wrong was going on.

"We have to get out." Mrs Sanford's voice rose to a near-scream. The panic in her voice was almost as frightening as the blood on the windows. "It's everywhere and if we don't get out we'll catch it too."

"She's right," the bus driver said, looking back at them. His hands clenched the steering wheel; they were shaking so hard the bus kept making little swerves.

Frankie glanced up and saw what he was seeing - more coughing, bloody-mouthed people staggering along the pavement. Several of them reeled towards the bus but it was going too fast for them to keep up.

"We can't just leave," she protested. "What about our families?"

It didn't matter then that hers were out of town for the weekend. She was thinking of Beth's gentle parents, and Allison's, whose over-protectiveness stemmed from love, and Melly's whose plump mother had given her daughter the ice cream-loving gene. They couldn't just leave everyone behind.

The driver shook his head. His eyes were bright with fear, blinking too rapidly. "Sorry, kid, we're getting out of here."

He spun the wheel and the bus took a sharp right. Frankie couldn't keep her balance and hit the floor.

"Take the slip road past Stanton Avenue then take the dual carriageway onto the M4. It's the quickest way out of Holmsley," Mrs Sanford said. Her voice was firmer now they had an escape route.

Several kids were crying, their shaky sobs filling the bus. Picking herself up, Frankie caught jumbles of words.

". . . don't understand . . ."

". . . but . . . our parents . . ."

". . . what's going on . . ."

Frankie was aware that Beth was clutching her again, her small fingers digging in. She automatically opened her mouth to comfort her friend but the words wouldn't come.

The bus crested a hill and turned off into Stanton Avenue, heading for the slip road that would take them out of town. The driver slowed to avoid a car that had been abandoned halfway across the road. It was the slowing down that saved their lives.

The next thing Frankie knew was a loud BANG and the bus veered out of control.