Octavia E. Butler's novel Clay's Ark is the prequel to her Clay Ark series. She wrote this one last, so it naturally feels a little disjointed from the rest of the series. Critics have agreed that the contents of the book seem somewhat hollow and the characters static, but the plot provides a satisfying explanation for the consequent developments in the other books of the series. Butler describes the space mission which became infected by alien microorganisms. The spaceship crash-landed in the Mojave Desert, Eli being the sole survivor. Driven mad by the urge to propagate the infection, he tries to contain the contagion by isolating his victims on a ranch in the desert. When the Maslin family escapes from the ranch -- already infected, -- they unwittingly spell the demise of the entire population. They do not possess Eli's mental fortitude to resist the biological urges of the symbiote, so they infect other people without containing the spread of the disease. One of the girls, however, Keira, chooses to willingly remain with Eli because the infection has cured her leukemia. She becomes one of the birth mothers of the clayarks.
Butler should be commended for most creative method of infection in a post-apocalyptic world. She has been criticizes, however, for the gruesome and violent contents of this book. In their urge to survive as a species, the alien microorganisms drive their hosts crazy to propagate. This leads to some rather horrific rapes, orgies, incest, etc. These people are literally being driven insane by the desire to have sex. In addition to these sex-crazed zombies, the clayark babies turn out to be mutant monsters. They have four legs and a taste for human flesh, infecting their victims by eating them.
As is typical with post-apocalyptic literature, this novel provides a fitting social commentary. Butler published this book in the 1970s, right when industry was creating a significant wage gap in America. She doubtless uses her personal observations of the economy to imagine a possible outcome to extremely unequal wealth distribution. Those who were fortunate divided into isolated communities of privileged but ignorant snobs. They deny the very existence of the lower class. For their part, the impoverished form tight-knit mobile family units and wander around stealing and attacking people. Each group seems to be living in a self-propagated delusion which is fed by their sheer ignorance and isolationism. People have adopted literal tribe mentalities, devolving to the habits of their nomadic ancestors.