I Am Legend stars Will Smith as Dr. Robert Neville, a virologist living in New York City with his dog Sam after a global apocalyptic event has nearly wiped out all of humanity. Robert spends his days collecting supplies, fortifying his home and continuing his studies to find a cure to the disease that has turned most of the population into hostile mutants. The film actually does address the early days of the outbreak and briefly explores the cause of this disease outbreak that Robert is attempting to cure. The cause actually differs from the original novel of the same name written by Richard Matheson, and below we will explore the two origins of the tragic disease.
I Am Legend Infection Source - Novel Edition
The basic premise of the film adaptation is essentially the same as the original novel. The novel is a heavy influence on both vampire and zombie literature and popular culture that followed, and there are three films based on the book including I Am Legend. In the novel, Robert Neville believes he is the last survivor of mankind on a mission to discover a cure for the disease that has turned most of the population into vampire-like monsters. These creatures have the markings of vampire mythos, including an aversion to garlic and sunlight.
Neville starts the novel in pure survival mode, hiding in his home at night while being terrorized by the infected. During the day, he's killing as many of the infected as he can by driving stakes through their hearts.
Eventually, he decides he must do more than simply survive and begins his mission to discover a cure. This mission leads him to discover that the root of the disease comes from a bacteria that similarly has an aversion to sunlight and garlic. Thus, the source of the infection in the novel is a natural mutation of bacteria that rapidly spread through humans.
I Am Legend Infection Source - 2007 Film Edition
The biggest liberty taken in the 2007 film adaptation of I Am Legend is undeniably the ending; however, there are other changes to the source material that take place even before then and that includes the actual source of the infection. To start, Robert Neville works for the United States military as a virologist. This makes him particularly prepared to search for a cure to the end of the world, but also, the film adjusts the actual source of the infection away from a naturally occurring mutation in bacteria. In the film, the source of the infection was instead man-made.
During the sequences set just prior to the film, and during the initial outbreak of the infection, Emma Thompson appears as Dr. Alice Krippin. Krippin has developed what she thinks is the cure for cancer, and this turns out to be a massive scientific breakthrough, though not in the way she, or the rest of humanity, hoped for. Instead of bringing the end to disease, it actually becomes the virus that infects the human race and turns them into the Darkseekers. Neville calls the disease the Krippin Virus as a result.
This cure for cancer was developed from the measles virus, but the new virus is unstable and mutates outside of its intended use. Instead of saving humanity, it gives the infected rabies-like symptoms and the infected become Darkseekers.
While some humans are immune to the Krippin virus, like Robert, most of the world's population are infected. Robert succeeds eventually in creating a cure, which he dies protecting and ensures its delivery to a survivors camp. While the source of the infection comes from good intentions, ultimately it nearly wiped out all of humanity.