Pictured: Intensive scientific research.
There's a heavy emphasis on how Morgan tries to deal with the psychological issues that come with the complete loneliness and despair of his situation. Morgan eventually encounters another survivor by the name of Ruth, but he soon learns that she's part of a group of infected people who suffer some of the effects of the disease but are able to hold the worst at bay with a vaccine. They're working to rebuild society, but they're terrified of Morgan since he appears to them as a mysterious killing machine (as he's taken down some of their kind during his career in vampire slaughtering).
Morgan escapes with Ruth's help, but he's eventually hunted down. In the novel he peacefully accepts his death, realizing that he's the last of his kind and the infected survivors represent the next step for humanity. The book's title, I Am Legend, referred to Morgan realizing that he, in his mass killings, had ironically become the mythical, nightmarish creature to this new breed.
The movie gets the same message across, but Morgan, half-crazy from years of isolation, goes out after a violent chase, eventually dying as angrily as possible. While it sounds like a huge change, the movie is otherwise pretty faithful, especially when compared to future attempts...
What the Remake Did Instead:
The first remake came out in 1971 under the name The Omega Man. Starring Charlton Heston, as Colonel Robert Neville, the monsters in this film are less scary than they are silly, with the vampires blaming the disease on the evils of science. So they're Luddite vampires, refusing to use anything other than the most primitive technologies. How do you think a bunch of bow and arrow wielding bad guys performed against the spokesperson for the NRA?
The movie mostly involves Neville shooting shit up, eventually coming across a group of people who are semi-resistant to the disease. He gives them a serum of his blood so they can become fully immune and restore humanity. He then dies heroically, knowing that he helped save the world from a plague of Amish vampires.