DOCTOR BY MURRAY LEINSTER Illustrated by FINLAY [Transcriber's Note: This etext was produced from Galaxy Magazine February 1961. Extensive research did not uncover any evidence that the U.S. copyright on this publication was renewed.] Suddenly the biggest thing in the universe was the very tiniest. There were suns, which were nearby, and there were stars which were so far away that no way of telling their distance had any meaning. The suns had planets, most of which did not matter, but the ones that did count had seas and continents, and the continents had cities and highways and spaceports. And people. The people paid no attention to their insignificance. They built ships which went through emptiness beyond imagining, and they landed upon planets and rebuilt them to their own liking. Suns flamed terribly, renting their impertinence, and storms swept across the planets they preëmpted, but the people built more strongly and were secure. Everything in the universe was bigger or stronger than the people, but they ignored the fact. They went about the businesses they had contrived for themselves. They were not afraid of anything until somewhere on a certain small planet an infinitesimal single molecule changed itself. It was one molecule among unthinkably many, upon one planet of one solar system among uncountable star clusters. It was not exactly alive, but it acted as if it were, in which it was like all the important matter of the cosmos. It was actually a combination of two complicated substances not too firmly joined together. When one of the parts changed, it became a new molecule. But, like the original one, it was still capable of a process called autocatalysis. It practiced that process and catalyzed other molecules into existence, which in each case were duplicates of itself. Then mankind had to take notice, though it ignored flaming suns and monstrous storms and emptiness past belief. Men called the new molecule a virus and gave it a name. They called it and its duplicates "chlorophage." And chlorophage was, to people, the most terrifying thing in the universe. * * * * * In a strictly temporary orbit around the planet Altaira, the _Star Queen_ floated, while lift-ships brought passengers and cargo up to it. The ship was too large to be landed economically at an unimportant spaceport like Altaira. It was a very modern ship and it made the Regulus-to-Cassim run, which is five hundred light-years, in only fifty days of Earthtime. Now the lift-ships were busy. There was an unusual number of passengers to board the _Star Queen_ at Altaira and an unusual number of them were women and children. The children tended to pudginess and the women had the dieted look of the wives of well-to-do men. Most of them looked red-eyed, as if they had been crying.