Horror/Post-Apocalyptic Book: The Book is set after an apocalypse and the main action takes place in a hospital. Main draw about the book is that its set in Houston and in a famous Nun-Hospital (Like St.Mary or something)

[Sun P. Persona]

I believe this book was in a 100 Horror Books of the Century or Encyclopedia of Horror. Besides that, I can't remember the rest of the plot due to not reading it. I was interested in reading it though.

The book was made around 1984-1990