question:  Why did Torp feel it was necessary to test Thig's blood for disease after he returned?
context: Thig carefully smoothed the dark sand and seaweed of the lonely beach over the metal lid of the flexible ringed tunnel that linked the grubby ship from another planet with the upper air. He stared appraisingly around at the luxuriant green growth of foliage further inland; and started toward the little stretch of trees and brush, walking carefully because of the lesser gravitation. Here he patiently sat down to wait for an Earthman or an Earthwoman to pass. His task now was to bring a native, intact if possible, back to the carefully buried space cruiser where his two fellows and himself would drain the creature's mentality of all its knowledge. In this way they could learn whether a planet was suited for colonization by later swarms of Orthans. Feebly he had punched a key two days ago and a $ sign had appeared. "There it is," announced Thig, dropping the limp body of the captured Earthman to the metal deck-plates. "With a psychic relay we will transfer the Earthman's memories and meager store of knowledge to the brain of Thig! "For the good of the Horde," Thig intoned almost piously as he lifted Terry's body and headed for the laboratory. So it was that the bodies of the Earthman and the Orthan were strapped on two parallel tables of chill metal and the twin helmets, linked to one another by the intricacies of the psychic relay, put upon their heads. Twice, with subtle drugs they restored pseudo-life to his body and kept the electrical impulses throbbing from his tortured brain, but after the third suspension of life Thig removed his helmet. An hour later, his scars and altered cartilage already healed and painless, Thig again scraped sand over the entrance to the space ship and set out along the moonlit beach toward the nearest path running inland to his home. Men had no mates on Ortha, sex had been overthrown with all the other primitive impulses of barbarism; so he was incapable of understanding the emotions that swept through his acquired memory.
output:  Thig had become sentimental over the people of Earth.
options:  Thig did not want to return to Ortha.  Thig seemed to be sick after he returned.  Thig had become sentimental over the people of Earth.  Thig's eyes were roaming and he seemed disoriented.