Sometimes it seemed to Doc that war was nothing but an endurance race to see how many times they could run before they were bombed. He was just beginning to drop off to sleep after a long trip for the sixth consecutive day when the little alarm shrilled. He sighed and shook Chris awake.

"Again?" she protested. But she got up and began helping him pack.

Jake came in, his eyes weary, pulling on the old jacket with the big star on its sleeve. Doc hadn't been too surprised to learn that Jake was the actual leader of the rebels. "Shuttles spotted taking off this way. And I still can't find where the leak is. They haven't missed our location once this week. Here, give me that."

He took the electron mike that had been among Doc's' possessions, but Chris recaptured it. "I can manage," she told him, and headed out for the tractor where Lou was waiting.

Doc scowled after her. He and Jake had been watching her. She was too useful to Doc's research to be turned away, but they didn't trust her yet. So far, however, they had found nothing wrong with her conduct. Still....

He swung suddenly into Jake's tractor. "Just remembered something. How'd they find me that time I stopped in the tractor to use the mike? I was pretty well hidden, and no tracks last in the sand long enough for them to have followed. But they were there when I came to. Somehow, they must have put a radio tracer on me."

Jake waited while they lighted up, his eyes suddenly bright. "You mean something you got from her house was bugged? It figures."

"And I've still got all the stuff. Now they find wherever we set up headquarters, though they've always managed to miss my laboratory, even when they've hit the troops around us. Jake, I think it's the microscope." Doc managed to push enough junk off one of the seats to make a cramped bed, and stretched out. "Sure, we figured they sent her because they want to keep tabs on what I discover. They've finally gotten scared of the plague, and she's the perfect Judas goat. But they have to have some way to get in touch with her. I'll bet there's a tracer in the mike and a switch so she can modulate it or key it to send out Morse."

"Yeah," Jake nodded. "Well, she does her own dirty work. I might get to like her if she was on our side. Okay, Doc. If they've put things into the mike, I've got a boy who'll find and fix it so she won't guess it's been touched."

Doc relaxed. For the moment, there would be no power in the instrument, nor any excuse for her to use it. But she must have handled some secret arrangement during the work periods. She used the mike more than he did. The switch could be camouflaged easily enough. If anyone detected the signal, they'd probably only think it was some leak in the electrical circuit.

Far away, the shuttle rockets had appeared as tiny dots in the sky. They were standing on their tails a second later, just off the ground, letting the full force of their blasts bake the area where headquarters had been.

Jake watched grimly, driving by something close to instinct. Then he looked back. "Know anything about a Dr. Harkness?"

"Not much, except that he protested sealing off the villages. Why?"

"He and five other doctors were picked up, trying to get through to us. Claimed they wanted to give us medical help. We can use them, God knows. I guess I'll have to chance it."

They stopped at a halfway village and hid the tractors before looking for a place to rest. Doc found Chris curled up asleep against the microscope. He had a hard time getting her to leave it in the tractor, but she was too genuinely tired to put up any real argument.

Jake reported in the morning before they set out again. "You were right, Doc. It was a nice job of work. Must have taken the best guys in Southport to hide the circuit so well. But it's safe now. It just makes a kind of meaningless static nobody can trace. Maybe we can get you a permanent lab now."

Doc debated again having Chris left behind and decided against it. The Lobby was determined to let him find a cure for them if he could. That meant Chris would work herself to exhaustion trying to help. Let her think she was doing it for the Lobby! It was time she was on the receiving end of a double cross.

"It's a stinking way to run a war," he decided.

Jake chuckled without much humor. "It's the war you wanted, remember? They forced our hand, but it had to come sometime. Right now the Lobby's fighting to get their hands on your work before we can use it; they're just using holding tactics, which helps our side. And we're hoping you get the cure so we can win. With that, maybe we'll whip them."

It was a crazy war, with each side killing more of its own men than of the enemy. The runners were increasing, and Jake's army was learning to shoot the poor devils mercifully and go on. They knew, at least, that there was no current danger of infection. In the Lobby towns, more were dying of panic in their efforts to escape the runners.

Desert towns had joined the villages, reluctantly but inevitably, to give the rebels nearly three-quarters of the total population. But the Lobby forces and the few cities held most of the real fighting equipment and they were ready to wait until Earth could send out unmanned rockets, loaded with atomics, which could cut through space at ten times normal speed.

There were vague lines of battle, but time was the vital factor. The Lobbies waited to steal a cure for the plague and the villages waited until they could announce it and demand surrender as its price.

It looked as if both sides were doomed to disappointment, however. He and Chris had put in every spare minute between moving and the minimum of sleep in searching for something that would check the disease. It couldn't grow in an Earth-normal body, but it didn't die, either. And there wasn't enough normal food available to permit the switch-over for more than a handful of people. Even Earth was out of luck, since eighty percent of her population ate synthetics. There were ways to synthesize Earth-normal food, but they were still hopelessly inefficient.

Jake had ordered one of the villages to rebuild their plant for such a purpose, while another was producing the enzyme that would permit switching. But it looked hopeless for more than a few of the most valuable men.

"No progress?" Jake asked for the hundredth time.

Doc grinned wryly. "A lot, but no help. We've found a fine accelerator for the bug. We can speed up its incubation or even make someone already infected catch it all over again. But we can't slow it down or stop it."

The new laboratory was still being fitted when they arrived. It had been dug into one of the few real cliffs in this section of Mars. The power plant had been installed, complete with a steam plant that would operate off sunlight in the daytime through a series of heat valves that took in a lot of warm air and produced smaller amounts hot enough to boil water.

"I'll see you whenever I can," Jake said. "But mostly, you're going to be somewhat isolated so they won't trace you. Let them think they goofed with the shuttles and hit you and Chris. Anything you need?"

"Guinea pigs," Doc told him sarcastically. It was meant as a joke, though a highly bitter one. Jake nodded and left them.

Doc opened the cots as Chris came in, not bothering to unpack the equipment. "Hit the sack, Chris," he told her.

She looked at him doubtfully. "You almost said that the way you'd address a human being, Dan. You're slipping. One of these days you'll like me again."

"Maybe." He was too tired to argue. "I doubt it, though. Forget it and get some sleep."

She watched him silently until he got up to turn out the light. Then she sighed heavily. "Dan?"

"Yeah?"

"I never got a divorce. The publicity would have been bad. But anyway, we're still married."

"That's nice." He swung to face her briefly. "And they found the radio in the microscope. Better get to sleep, Chris."

"Oh." It was a quiet exclamation, barely audible. There was a sound that might have been a sniffle if it had come from anyone else. Then she rolled over. "All right, Dan. I still want to help you."

He cursed himself for a stupid fool for telling her. Fatigue was ruining what judgment he had. From now on, he'd have to watch her every minute. Or had she really seen the value of the research by now? She wasn't a fool. It should have registered on even her stubborn mind. But he was too sleepy to think about it.

She had breakfast ready in the morning. She made no comment on what had been said during the night. Instead, she began discussing a way to keep one of the organic antibiotics from splitting into simpler compounds when they tried to switch it over to Mars-normal. They were both hopelessly bad chemists and biologists, but there was no one else to do the work.

Chris worked harder than ever during the day.

Just after sundown, Jake came in with a heavy box. He dropped it onto the floor. "Mice!"

Doc ripped off the cover, exposing fine screening. There were at least six dozen mice inside!

"Harkness found them," Jake explained. "A hormone extraction plant used them for testing some of the products. Had them sent by regular shipments from Earth. Getting them cost a couple of men, but Harkness claims it's worth it. He's a good man on a raid. Here!"

He'd gone to the doorway again and came back with another box, this one crammed with bottles and boxes. "They had quite a laboratory, and Harkness picked out whatever he thought you could use."

Chris and Doc were going through it. The labels were engineering ones, but the chemical formulae were identification enough. There were dozens of chemicals they hadn't hoped to get.

"Anything else?" Doc finally asked as they began arranging the supplies.

"More runners. A lot more. We're still holding things down, but it's reaching a limit. Panic will start in the camps if this keeps on. But that's my worry. You stick to yours."

Several of the new chemicals showed promise in the tubes. But two of them proved fatal to the mice and the others were completely innocuous in the little animal's bodies, both to mouse and to germ. The plague was much hardier in contact with living cells than in the artificial environment of the culture jars.

They lost seven mice in two days, but that seemed unimportant; the females were already living up to their reputations, nearly all pregnant. Doc didn't know the gestation period, but he remembered that it was short.

"Funny they all started at the same time," he commented. "Must have been shipped out separately or else been kept apart while they were switched over to Mars-normal. Something interrupted their habits, anyhow."

A few nights later they learned what it was. There was a horrible squealing that woke him out of the depths of his sleep. Chris was already at the light switch. As light came on, they turned to the mouse box.

All the animals were charging about in their limited space, their little legs driving madly and their mouths open. What they lacked in size they made up in numbers, and the din was terrific.

But it didn't last. One by one, the mice began dropping to the floor of the cage. In fifteen minutes, they were all dead!

It was obviously the plague, contracted after having their metabolism switched. Women were sterile for some time after Selznik's migraine struck, and the same must have been true of the mice. They must have contracted the plague at about the same time and reached fertility together. Somehow, the plague incubation period had been shortened to fit their life span; the disease was nothing if not adaptive.

Chris prepared a slide in dull silence. The familiar cell was there when Doc looked through the microscope. He picked up one of the little creatures and cut it open, removing one of the foetuses.

"Make a film of that," he suggested.

She worked rapidly, scraping out the almost microscopic brain, dissolving out the fatty substance, and transferring the result to a film. This time, even at full magnification, there was no sign of the filaments that were always present in diseased flesh. The results were the same for the other samples they made.

"Something about the very young animal or a secretion from the mother's organs keeps the bug from working." Doc reached for a bracky weed and accepted a light from Chris without thinking of it. "Every kid I've heard about contracted the plague between the second and third year. None are born with it, none get it earlier. I've suspected this, but now here's confirmation."

Chris began preparing specimens, while Doc got busy with tubes of the culture. They'd have to test various fluids from the tiny bodies, but there were enough cultures prepared. Then, if the substance only inhibited growth, there would be a long, slow test; if it killed the bugs, they might know more quickly.

Jake came in before the final tests, but waited on them. Doc was studying a film in the microscope. He suddenly motioned excitedly for Chris.

"See the filaments? They're completely disintegrated. And there's one of the big cells broken open. We've got it! It's in the blood of the foetus. And it must be in the blood of newborn children, too!"

Jake looked at the slide, but his face was doubtful.

"Maybe you've got something, Doc. I hope so. And I hope you can use it." He shook his head wearily. "We need good news right now. A couple of big rockets just reached the station and they've been sending shuttles back and forth a mile a minute. Nobody can figure how they got here so fast or what they're for. But it doesn't look good for us!"