These four books that comprise the Patternmaster books can definitely be read in any order, as the earliest published book is the last chronologically, and this book, which was the last published is the third in the series, and she even refused to renew the copyright of another book (which you can find online if you like).
In this book we begin with a violent carjacking in a kind of wasteland area of the US. A father and two daughters, both teenage, and one seemingly dying of cancer, are saved from the carjacking by a small band who then take them back to their enclave. In the enclave they are told they either have been or will be infected in a parasite, which works like a virus, but is actually an alien presence that begins to reshape their DNA. Kind of a blend of rabies and toxoplasmosis by prognosis, in that the host begins to look for danger and sex in order to spread the infection, but that works to enhance human abilities in both progressive and seemingly atavistic ways. The enclave exists to guide people through the process and try to keep it contained. The disease, of course, does not want this.
We learn more about this mysterious ailment and it should remind us of some things we might have learned about in the fourth novel, Patternmaster.