"The Omega Man" (1971) with Charlton Heston.
I like it much more than Will Smith's "I Am Legend." I've never read the book, so I no idea which one is more faithful to the book, but I like the fact that the infected people in "The Omega Man" aren't mindless zombies. In fact, the leader of the infected, Matthias (played brilliantly by Anthony Zerbe), is an intelligent, charismatic leader, and one of my favorite movie villains.
I've read the book quite a while ago and don't remember many details, but I think the zombies there mostly weren't villains at all. A protagonist turned out to be the biggest villain in the end.