I recently read the SF novel "The Last Mortal Man" by Syne Mitchell. It's billed as the first installment of a series called "The Deathless."
One of the major characters is Alexa DuBois, an African-American woman who works as an elite bodyguard for the billionaire (Lucius Sterling) who controls the process that replaces a normal human body with an "immortal" body made out of some artificially-created compounds which have serious advantages over the molecules that make up normal human flesh and blood. You might call Alexa a slave -- she has accepted a 150-year "indenture" to serve Lucius or his heirs, in one of those state-of-the-art immortal bodies with superhuman abilities, before she will finally be allowed to quit her job and go do whatever else she wants to do with the rest of her life. (Even with her artificial body, it is possible for her to die in the line of duty, so there's no telling whether or not she will make it to the 150-year retirement date. After all, the story wouldn't be very suspenseful if we knew she was guaranteed to survive every emergency that came along.)
One of the major plot points in the book is that someone has invented a sort of bioweapon that dissolves anything made out of "nanobiology" materials, which has come to mean (a few centuries from now) the vast majority of clothes, vehicles, buildings, and so on and so forth. One strike by this weapon can cause a good chunk of a city to sort of melt away (including any nanobiological enhancements which have been added to a previously-normal human body). However, the weapon does not spread uncontrollably, like a virus -- instead, it works for a certain amount of time, in a certain area, and then I believe it automatically goes inert as a safety feature.
It sounds very much as if you might be remembering this book. I wondered if you might have read a later book in the series -- I don't remember Alexa falling madly in love with some other enhanced-soldier guy during the first volume, although she did have an adversary who had the same sort of advantages that she did -- but just now I did some Googling, and I learned, to my surprise, that no sequels to the first book have ever been published. In fact, it looks like "The Last Mortal Man," published in 2006, was Syne Mitchell's last published novel (according to Amazon.com and the ISFDB). I hadn't realized that -- I'd been thinking I'd want to pick up the second volume in the series, one of these days!