Quite suddenly the little black eyes closed and Fuzzy began to tremble violently. He drew himself up into a tight pink globule as the fuzz-like hair disappeared from view. Something was unmistakably wrong. As he held the shivering creature, Dal was suddenly aware that something had been nibbling at the back of his mind for hours. Not a clear-cut thought, merely an impression of pain and anguish and sickness, and now as he looked at Fuzzy the impression grew so strong it almost made him cry out. Abruptly, Dal knew what he had to do. Where the thought came from he didn't know, but it was crystal clear in his mind. "Jack, where is our biggest virus filter?" he asked quietly. Jack stared at him. "Virus filter? I just took it out of the autoclave an hour ago." "Get it," Dal said, "and the suction machine too. _Quickly!_" Jack went down the corridor like a shot, and reappeared a moment later with the big porcelain virus filter and the suction tubing attached to it. Swiftly Dal dumped the limp little creature in his hand into the top of the filter jar, poured in some sterile saline, and started the suction. Tiger and Jack watched him in amazement. "What are you doing?" Tiger said. "Filtering him," Dal said. "He's infected. He must have been exposed to the plague somehow, maybe when our little Bruckian visitor came on board the other day. And if it's a virus that's causing this plague, the virus filter ought to hold it back and still let Fuzzy's molecular structure through." They watched and sure enough a bluish-pink fluid began moving down through the porcelain filter, and dripping through the funnel into the beaker below. Each drop coalesced in the beaker as it fell until Fuzzy's whole body had been sucked through the filter and into the jar below. He was still not quite his normal pink color, but as the filter went dry, a pair of frightened shoe-button eyes appeared and he poked up a pair of ears. Presently the fuzz began appearing on his body again. And on the top of the filter lay a faint gray film. "Don't touch it!" Dal said. "That's real poison." He slipped on a mask and gloves, and scraped a bit of the film from the filter with a spatula. "I think we have it," he said. "The virus that's causing the plague on this planet." CHAPTER 10 THE BOOMERANG CLUE It was a virus, beyond doubt. The electron microscope told them that, now that they had the substance isolated and could examine it. In the culture tubes in the _Lancet_'s incubators, it would begin to grow nicely, and then falter and die, but when guinea pigs were inoculated in the ship's laboratory, the substance proved its virulence. The animals injected with tiny bits of the substance grew sick within hours and very quickly died.