What book is the 1971 film "The Omega Man" based on?

"The Omega Man" sprang from Richard Matheson's groundbreaking 1954 novel "I Am Legend," which pioneered the concept of a worldwide pandemic turning humans into vampire-like creatures. Hollywood has adapted the story three times - with Vincent Price in 1964's "The Last Man on Earth," Charlton Heston battling mutants in "The Omega Man," and Will Smith hanging out with mannequins in 2007's "I Am Legend." None of the films really captured Matheson's original story, but they sure had fun trying.