"There was. Those specimens came from somewhere beyond Regulus. They carried, as normal symbiotes on their foliage, microörganisms unknown both on Kamerun and Altaira. The alien bugs are almost the size of virus particles, feed on virus particles, and are carried by contact, air, and so on, as readily as virus particles themselves. We discovered that those microörganisms devoured chlorophage. We washed them off the leaves of the plants, sprayed them in our air-room jungle, and they multiplied faster than the chlorophage. Our whole air supply is now loaded with an airborne antichlorophage organism which has made our crew and passengers immune. We're heading back to Altaira to turn loose our merry little bugs on that planet. It appears that they grow on certain vegetation, but they'll live anywhere there's phage to eat. We're keeping some chlorophage cultures alive so our microörganisms don't die out for lack of food!" The medical officer on the ground gasped. "Keeping phage _alive_?" * * * * * "I hope you've recorded this," said Nordenfeld. "It's rather important. This trick should have been tried on Kamerun and Altaira and everywhere else new diseases have turned up. When there's a bug on one planet that's deadly to us, there's bound to be a bug on some other planet that's deadly to it! The same goes for any pests or vermin--the principle of natural enemies. All we have to do is find the enemies!" There was more communication between the _Star Queen_ and the spaceport on Circe III, which the _Star Queen_ would not make other contact with on this trip, and presently the big liner headed back to Altaira. It was necessary for official as well as humanitarian reasons. There would need to be a health examination of the _Star Queen_ to certify that it was safe for passengers to breathe her air and eat in her restaurants and swim in her swimming pools and occupy the six levels of passenger cabins she contained. This would have to be done by a Patrol ship, which would turn up at Altaira. The _Star Queen's_ skipper would be praised by his owners for not having driven the liner into a star, and the purser would be forgiven for the confusion in his records due to off-schedule operations of the big ship, and Jensen would find in the ending of all terror of chlorophage an excellent reason to look for appreciation in the value of the investments he was checking up. And Doctor Nordenfeld.... He talked very gravely to Kathy. "I'm afraid," he told her, "that your father isn't coming back. What would you like to do?" She smiled at him hopefully. "Could I be your little girl?" she asked. Doctor Nordenfeld grunted. "Hm ... I'll think about it." But he smiled at her. She grinned at him. And it was settled. End of the Project Gutenberg EBook of Doctor, by Murray Leinster