Link to interview with Richard Matheson conducted for the unreleased book The Last Man on Film: Adaptations of Richard Matheson's I Am Legend:
http://iamlegendarchive.blogspot.com/p/richard-matheson-interview.html
Richard Matheson: "The idea for I Am Legend came to me when I was about 16. I went to see Dracula and the thought occurred that, if one vampire was scary, a world filled with vampires would really be scary. I did not write the book until 1952. We lived in Gardena, California and I set the story there, using our house as Neville's house. I think that ascribing metaphors to a book after it is written is silly. My son Richard provided a much more likely one- that it was prophetic because of AIDS. I don't think the book means anything more than it is: the story of a man trying to survive in a world of vampires. If people want to assume it later, that's up to them. It has been said that a writer is entitled to an interpretation of his work that people choose to give it."
Much more at link:
http://iamlegendarchive.blogspot.com/p/richard-matheson-interview.html