Alex, as I understand it, in the book Neville becomes the titular "Legend" because he is the boogeyman of the new civilzation of albino, sun-allergic creatures that inherit the earth as the dominant species after the demise of 20th century homosapians. Supposedly, it's a neat twist that our hero is revealed to be the bad guy from the only perspective that counts once he's dead. Is that right?
In the film, he's not the last man alive. He gives a blood sample of his resistent plasma to a woman and boy he has hide in a crawl space from albino attackers, and then he is killed. There is no explanation why Neville himself could not hide in the crawl space, too ... and thus live to provide far more than a blood sample to science. But in the film, it's his blood sample that provides the cure, and thus He Is Legend.