"If you'll wait a moment-One by one, after my experiments had reached the point where I felt the instrument might be safely applied to human beings, certain of Earth's biologists were treated. In each case they were men I knew to be in sympathy with the Zealots-the extremists, that is. They all survived, though secondary effects made themselves shown after a time. One of them was brought back for treatment eventually. I could not save him. But, in his dying delirium, I found out."

It was close upon midnight. The day had been long and much had happened. But now something stirred within Arvardan. He said tightly, "I wish you'd get to the point."

Shekt said, "I beg your patience. I must explain thoroughly, if you're to believe me. You, of course, know of Earth's peculiar environment-its radioactivity-"

"Yes, I have a fair knowledge of the matter."

"And of the effect of this radioactivity upon Earth and its economy?"

"Yes."

"Then I won't belabor the point. I need only say that the incidence of mutation on Earth is greater than in the rest of the Galaxy. The idea of our enemies that Earthmen are different thus has a certain basis of physical truth. To be sure, the mutations are minor, and most possess no survival value. If any permanent change has occurred in Earthmen, it is only in some aspects of their internal chemistry which enables them to display greater resistance to their own particular environment. Thus they show greater resistance to radiation effects, more rapid healing of burned tissues-"

"Dr. Shekt, I am acquainted with all you say."

"Then has it ever occurred to you that these mutational processes occur in living species on Earth other than human?"

There was a short silence, and then Arvardan said, "Why, no, it hasn't, though, of course, it is quite inevitable, now that you mention it."

"That is so. It happens. Our domestic animals exist in greater variety than on any other inhabited world. The orange you ate is a mutated variety, which exists nowhere else. It is this, among other things, which makes the orange so unacceptable for export. Outsiders suspect it as they suspect us-and we ourselves guard it as a valuable property peculiar to ourselves. And of course what applies to animals and plants applies also to microscopic life."

And now, indeed, Arvardan felt the thin pang of fear enter.

He said, "You mean-bacteria?"

"I mean the whole domain of primitive life. Protozoa, bacteria, and the self-reproducing proteins that some people call viruses. "

"And what are you getting at?"

"I think you have a notion of that, Dr. Arvardan. You seem suddenly interested. You see, there is a belief among your people that Earthmen are bringers of death, that to associate with an Earthman is to die, that Earthmen are the bearers of misfortune, possess a sort of evil eye-"

"I know all that. It is merely superstition."

"Not entirely. That is the dreadful part. Like all common beliefs, however superstitious, distorted, and perverted, it has a speck of truth at bottom. Sometimes, you see, an Earthman carries Within his body some mutated form of microscopic parasite which is not quite like any known elsewhere, and to which, sometimes, Outsiders are not particularly resistant. What follows is simple biology, Dr. Arvardan. "

Arvardan was silent.

Shekt went on, "We are caught sometimes, too, of course. A new species of germ will make its way out of the radioactive mists and an epidemic will sweep the planet, but, by and large, Earthmen have kept pace. For each variety of germ and virus, we build our defense over the generations, and we survive. Outsiders don't have the opportunity."

"Do you mean," said Arvardan with a strangely faint sensation, "that contact with you now-" He pushed his chair back. He was thinking of the evening's kisses.

Shekt shook his head. "Of course not. We don't create the disease; we merely carry it. And even such carriage occurs very rarely. If I lived on your world, I would no more carry the germ than you would; I have no special affinity for it. Even here it is only one out of every quadrillion germs, or one out of every quadrillion of quadrillions, that is dangerous. The chances of your infection right now are less than that of a meteorite penetrating the roof of this house and hitting you. Unless the germs in question are deliberately searched for, isolated, and concentrated."

Again a silence, longer this time. Arvardan said in a queer, strangled voice, "Have Earthmen been doing that?"

He had stopped thinking in terms of paranoia. He was ready to believe.

"Yes. But for innocent reasons, at first. Our biologists are, of course, particularly interested in the peculiarities of Earth life, and, recently, isolated the virus of Common Fever."

"What is Common Fever?"

"A mild endemic disease on Earth. That is, it is always with us. Most Earthmen have it in their childhood, and its symptoms are not very severe. A mild fever, a transitory rash, and inflammation of the joints and of the lips, combined with an annoying thirst. It runs its course in four to six days, and the subject is thereafter immune. I've had it. Pola has had it. Occasionally there is a more virulent form of this same disease-a slightly different strain of virus is concerned, presumably-and then it is called Radiation Fever."

"Radiation Fever. I've heard of it," said Arvardan.

"Oh, really? It is called Radiation Fever because of the mistaken notion that it is caught after exposure to radioactive areas. Actually, exposure to radioactive areas is often followed by Radiation Fever, because it is in those areas that the virus is most apt to mutate to dangerous forms. But it is the virus and not the radiation which does it. In the case of Radiation Fever, symptoms develop in a matter of two hours. The lips are so badly affected that the subject can scarcely talk, and he may be dead in a matter of days.

"Now, Dr. Arvardan, this is the crucial point. The Earthman has adapted himself to Common Fever and the Outsider has not. Occasionally a member of the Imperial garrison is exposed to it, and, in that case, he reacts to it as an Earthman would to Radiation Fever. Usually he dies within twelve hours. He is then burned-by Earthmen-since any other soldier approaching also dies.

"The virus, as I say, was isolated ten years ago. It is a nucleoprotein, as are most filtrable viruses, which, however, possesses the remarkable property of containing an unusually high concentration of radioactive carbon, sulphur, and phosphorus. When I say unusually high I mean that fifty per cent of its carbon, sulphur, and phosphorus is radioactive. It is supposed that the effects of the organism on its host is largely that of its radiations, rather than of its toxins. Naturally it would seem logical that Earthmen, who are adapted to gamma radiations, are only slightly affected. Original research in the virus centered at first about the method whereby it concentrated its radioactive isotopes. As you know, no chemical means can separate isotopes except through very long and tedious procedures. Nor is any organism other than this virus known which can do so. But then the direction of research changed.

"I'll be short, Dr. Arvardan. I think you see the rest. Experiments might be conducted on animals from outside Earth, but not on Outsiders themselves. The numbers of Outsiders on Earth were too few to allow several to disappear without notice. Nor could premature discovery of their plans be allowed. So it was a group of bacteriologists that was sent to the Synapsifier.. to return with insights enormously developed. It was they who developed a new mathematical attack on protein chemistry and on immunology, which enabled them finally to develop an artificial strain of virus that was designed to affect Galactic human beings-Outsiders-only. Tons of the crystallized virus now exist."