Reproduction
The alien microorganisms are motivated by the survival of their species. When they infect the astronauts, they take charge of their reproductive systems causing a mass orgy. They're only concern is propagating the species. Back on Earth, Eli gives way to these sexual urges by kidnapping and raping a number of people whom he sets up in his desert ranch. Each of his victims become infected and suffer from similar urges. =Finally, the clayark mutants -- babies of Eli and his victims -- are born with the same insatiable desires. They spread their infections by feasting on uninfected people, however. Now there are hungry four-legged zombies and sex-crazy zombies running around the southwest.
Uneven Wealth Distribution
In this dystopian world, the wage gape has become so intense as to create a polarized economy. Some of the population lives life as normal, possessing immense wealth. They have shut themselves up in gated communities and all but denied the existence of anyone else. Meanwhile the poor of the nation have formed nomadic tribes called car families. They are brutal people who ensure loyalty with blood. Their lifestyles could not be more different from the privileged few. In a world where there is no communication between the classes, these people have come to exist in entirely separate spheres.
Resistance
Willpower plays an interesting role in this story. Eli could've returned to Earth and immediately succumbed to his primordial urges from the alien infection, but he puts up a fight. At first he contents himself with a handful of victims whom he keeps isolated. Eventually his desires become more than he can handle, so he establishes this ranch community for the infected, trying to prevent the spread of the microorganisms. Although he is not in control of his brain, Eli tries to resist the urges of his infection as long as possible. He is motivated by the same thing as the micro-organisms, after all -- survival of the species. He is unwilling to be responsible for the destruction of humanity. On the other hand, the alien lifeforms feel the same way about their own species, eager to take advantage of their vulnerable new hosts.
Illness
While Butler was writing this series, she sent recently completed manuscripts to her friend who was dying of cancer. Her fixation upon disease and illness is obvious. After all the aliens first surface as microbes. Only after infecting their human hosts with the bacteria do they propagate a macro species. The clayarks of the product of two infected hosts. This means the prevention of the spread of the infection depends upon normal health precautions to prevent the spread of disease. Interestingly enough, Keira's leukemia is cured by the microorganisms. She finds redemption at Eli's ranch, proudly taking her place as a clayark birth mother.
