As a child, Sandoval suffered from lobar pneumonia, which scarred the tissue on his lungs. As well, he had an appendectomy and received the required inoculations. In 2263, these facts were included in his medical report before he left Earth on an expedition to Omicron Ceti III.
In 2263, he and 150 other colonists departed Earth for Omicron Ceti III hoping to establish an agricultural colony where they could "return to a less complicated life." After landing on the planet in 2264, Sandoval and the others were taken over by spores that protected them from the deadly berthold radiation bombarding the planet. The spores provided a constant feeling of contentment, and also perfect physical health. Sandoval's lungs were free of the scarred tissue and his appendix was restored.
When the USS Enterprise investigated the Omicron colony in 2267, Sandoval conspired with colony botanist Leila Kalomi to infect the starship with the spores. Though he was initially successful in converting the entire crew to his colony, Enterprise Captain James T. Kirk used a subsonic transmitter to destroy the spores. Freed of their influence, Sandoval realized he had accomplished nothing he had set out to do for the past three years. He planned to try to start a new colony once the Enterprise evacuated the Omicron colonists to Starbase 27. (TOS: "This Side of Paradise")