Complex sentence: But the aliens must overcome humanity's susceptibility to its diseases, and the willpower of mankind's resistance to slavery.They discover that if they survive the humid, viral climate, the immobile Luminoids will abduct Earth's remaining population for enslavement.
Split into simpler sentences: But the aliens must overcome humanity's own susceptibility to that disease, and the willpower of mankind's resistance to slavery. The abductees soon discover that if they survive the humid, viral climate, the immobile Luminoids will abduct Earth's remaining population for enslavement.