"Look, I've seen a dozen articles showing how such a thing was theoretically possible ... a virus life-form with billions of submicroscopic parts acting together to form an intelligent colony. The only thing a virus-creature would need that other intelligent creatures don't need would be some kind of a host, some sort of animal body to live in so that it could use its intelligence." "It's impossible," Jack said scornfully. "Why don't you give it up and get some rest? Here we sit with our feet in the fire, and all you can do is dream up foolishness like this." "I'm not so sure it's foolishness," Tiger Martin said slowly. "Jack, maybe he's got something. A couple of things would fit that don't make sense at all." "All sorts of things would fit," Dal said. "The viruses we know have to have a host--some other life-form to live in. Usually they are parasites, damaging or destroying their hosts and giving nothing in return, but some set up real partnership housekeeping with their hosts so that both are better off." "You mean a symbiotic relationship," Jack said. "Of course," Dal said. "Now suppose these virus-creatures were intelligent, and came from some other place looking for a new host they could live with. They wouldn't look for an intelligent creature, they would look for some _unintelligent_ creature with a good strong body that would be capable of doing all sorts of things if it only had an intelligence to guide it. Suppose these virus-creatures found a simple-minded, unintelligent race on this planet and tried to set up a symbiotic relationship with it. The virus-creatures would need a host to provide a home and a food supply. Maybe they in turn could supply the intelligence to raise the host to a civilized level of life and performance. Wouldn't that be a fair basis for a sound partnership?" Jack scratched his head doubtfully. "And you're saying that these virus-creatures came here after the exploratory ship had come and gone?" "They must have! Maybe they only came a few years ago, maybe only months ago. But when they tried to invade the unintelligent creatures the exploratory ship found here, they discovered that the new host's body couldn't tolerate them. His body reacted as if they were parasitic invaders, and built up antibodies against them. And those body defenses were more than the virus could cope with."