Bob Andrews made a beeline across the Jones Salvage Yard to the headquarters trailer of The Three Investigators. He was swinging a tennis racket and mentally juggling his heavy schedule.

Spring vacation had just started, and Bob had plans for every minute of it. Right now he needed to squeeze in some time on an overdue history report before keeping a late-afternoon tennis date. Then that evening he had to set up a rock band for Sax Sendler, his boss at Rock-Plus talent agency. Lucky that The Three Investigators had no cases at the moment!

In the grease pit next to the trailer, Bob’s fellow Investigator Pete Crenshaw whistled happily to himself. Most of Pete had disappeared beneath the hood of the beat-up van he was repairing. He expected to make a bundle this week when he finished fixing up the van and sold it. Pete didn’t want any detective work this week either.

Just then a speeding truck roared in through the open gates of the junkyard. Bob looked around in surprise. Pete jumped up from beneath the hood, almost cracking his head. Who was in such a big hurry?

The truck was one of the yard’s own battered pick-ups. Brakes squealing, it halted in a cloud of dust. Browsing customers scattered in alarm. Wild drivers were rare in the biggest and best junkyard in Rocky Beach, California.

“Hey, it’s Jupe!” Bob yelled to Pete.

“What’s he up to now?” Pete shouted back.

Behind the wheel, the round face of Jupiter Jones was creased with worry. Jupe turned off the engine, jumped out of the pickup, and dashed wordlessly past Bob and Pete toward the trailer. His black hair was mussed, as usual, but his face was ghostly white… Next to starvation, exercise was Jupe’s least favorite thing. When stout Jupe ran, Pete and Bob knew something was up. And by the look on Jupe’s face, this time it was something really bad.

“Jupe! What gives?” called Pete. Wiping his hands on his green Yosemite T-shirt, he chased after him. At six feet one and well-coordinated pounds, Pete could, move fast.

“Who’s after you, Jupe?” Bob yelled, running to catch up. Though not a jock like Pete, the tall, slim, blond guy could move plenty fast himself.

Jupe’s broad backside disappeared into the trailer. Pete and Bob pounded in behind him.

The headquarters of the three seventeen-year-olds was its usual jumble. Papers and empty fast-food containers littered the tops of the desk and file cabinet.

The faint odor of yesterday’s anchovy pizza hung in the air.

As the door banged shut, Jupe whipped off the cover to the Investigators’ PC, Not only was he the brains of the guys’ amateur detective agency, he was also an electronics whiz and a hotshot computer programmer. While Bob and Pete watched, Jupe sat down at the computer, flipped switches, and slid two floppy disks into their slots. The PC whirred quietly.

Jupe took a deep breath and stared tensely at the PC.

“Come on, Jupe,” Pete said. “Spill it!” Bob said.

Jupe shook his head and gestured for them to bug off. He wasn’t ready to talk yet.

Bathed in the glow of the PC’s monitor, the Investigators stared as bright amber letters moved across the black computer screen. Jupe typed a request for the computer to show some information.

Bob was growing impatient. “Speed it up, Jupe. I need the PC so I can finish up my history report. The girls will be here any sec.” He flicked the strings of his tennis racket. He and Pete had a tennis date with Elizabeth Zapata and Kelly Madigan. Bob had just met Elizabeth at a downtown record store, and Kelly was Pete’s steady.

“Yikes!” Pete whooped. “Kelly!” He looked down at his T-shirt and jeans. They were streaked with black automotive grease and white primer paint. “I’ve got to change!”

As Pete bounded off toward his pile of clean clothes at the back of the trailer, the PC gave a sharp, high-pitched beep. Instantly three huge words flashed onto the screen: FATAL DISK ERROR!

“What’s that mean?” Bob demanded suspiciously.

“The pits,” Jupe groaned. “We can’t get our information off the disk. But even worse, it’s what I was afraid of… The virus has infected us, too!”

“What virus?” Pete demanded from the back of the trailer. He’d stripped off his T-shirt and was cleaning his hands and arms with naval jelly. “I feel fine.”

“Not a people virus, Pete,” Jupiter growled. His fingers flew over the keyboard again, trying to convince the computer to find some of the information on the disk. “A computer virus.”

Puzzled, Pete returned to stand over Jupe. Each time Jupe tried a new command, the PC beeped ominously, and FATAL DISK ERROR glared out at the guys.

“What’s a computer virus?” Pete finally asked.

“A tiny string of code that copies itself over and over,” Jupiter answered grimly. “It’s really a subprogram that hides inside another computer program or an operating system. Then it eats and scrambles data.”

“Run that by me again,” Pete said, even more puzzled.

“The virus erases information,” Jupe translated. “Or it screws it up so that you don’t know if it’s accurate anymore.”

“You mean it can change numbers and words?” Bob said.

“That’s it,” said Jupe. “Viruses are the worst. Some fill a disk with many nonsense numbers that the computer crashes. Others just randomly ruin data. The damage possibilities are endless.”

“But Jupe, how come we didn’t know we had a virus until now?” Bob asked.

“Do you know the moment you catch the flu?” Jupe asked.

“No.” Bob and Pete shook their heads.

“Same thing happens with computer viruses. It all depends on the design. They can screw up your data right away, or they can sit around for months and then whammo!” Jupe slammed a fist into the palm of his hand. “They spread from one disk to another through the computer. So if you borrow someone else’s disk, you’d better watch out. Your computer might catch one!”

“Wait’ll I tell my math teacher,” Pete said. “Numbers are contagious.”

“I’d like to make this contagious!” Jupe said, shaking his fist. “The jerk who designed this virus probably thinks he’s hot stuff right now. But we’ve lost everything on this game disk. The files are wiped clean!”

Suddenly Bob felt queasy. He grabbed the disk box and handed it to Jupe. “I’ve put blood, sweat, and even a few broken dates into my history report. Tell me it’s okay!”

Jupe turned off the computer and started it again. He slid in Bob’s disk.

Bob leaned close to the screen and held his breath. Jupe called up the report file.

Bob’s luck was the worst: Beep. FATAL DISK ERROR!

“It’s gone,” Bob moaned. “I don’t believe it. Gone! I’ll have to write the whole thing all over again. It was fifteen pages long!”

“Sorry, guy.” Pete clapped Bob on the shoulder and pulled up chairs for the two of them. “What can we do, Jupe?”

Jupiter was hard at work. “This is a sector editor,” he explained as he waved a disk. “I bought it on the way home. It’s like a high-powered magnifying glass. Watch this.” He switched disks in the computer.

But Bob stood up and walked away. Pete glanced in his direction and saw him searching through a pile of papers on the bookcase. He looked weirded out.

Pete leaned back over Jupe’s shoulder.

“The sector editor lets us look at the data on the disk’s rings,” Jupe said. “There are a lot of rings — three hundred sixty concentric ones on each disk.”

“Like the rings around a CD?”