encapsulated sons

Driven by over-population, a pure culture of the virus-creatures had long ago departed from their original native hosts, and traveled like encapsulated spores across space from a distant galaxy. The trip had been long and exhausting; the virus-creatures had retained only the minimum strength necessary to establish themselves in a new host, some unintelligent creature living on an uninhabited planet, a creature that could benefit by the great intelligence of the virus-creatures, and provide food and shelter for both. Finally, after thousands of years of searching, they had found this planet with its dull-minded, fruit-gathering inhabitants. These creatures had seemed perfect as hosts, and the virus-creatures had thought their long search for a perfect partner was finally at an end.

[STAR SURGEON by ALAN E. NOURSE - 1959]