The plague bomb functioned as a biological warfare tool, deployed during a conflict situated in a galaxy vastly removed from the galaxy we know.
During a war that unfolded in a far-off galaxy, the plague bomb served the purpose of eradicating the inhabitants of the home planet belonging to Ship's pilot. After Ship's apparent victory, having vanquished the final opposing mecho-organic vessel and ostensibly secured victory in the war, Ship's crew discovered their planet ravaged, and every one of them, save for the pilot, perished from the plague before they could make it back to the ship. As far as he could tell, the pilot stood as the sole survivor of his species to withstand the plague.