Francis Lawrence's 2007 post-apocalyptic sci-fi movie I Am Legend takes place in a near-future where a virus designed to cure cancer has wiped out most of humanity instead. Left behind are a few stray survivors fortunate enough to be immune to the airborne virus, and the people who have been mutated by the virus into savage monsters called Darkseekers.
I Am Legend is based on the novel of the same name by Richard Matheson, which was published in 1954. Though many changes were made from the original novel, the protagonist remains the same: Robert Neville, a sole survivor of the pandemic who is attempting to use science better understand the creatures it has created - and perhaps even find a cure. Robert is played by Will Smith in I Am Legend, and for most of the movie he is alone with only his pet dog, Sam, for company.
Robert is one of the few people who are immune to the virus, which is called the Krippin virus after its creator, Dr. Alice Krippin (Emma Thompson). He insists upon remaining in the ruins of New York City, since it was ground zero for the outbreak, and has spent years attempting to synthesize a cure for the Krippin virus using his own blood as the basis for his compounds. But what exactly is the Krippin virus, how does it spread, and how was it created?
I Am Legend's Krippin Virus Explained
A news interview with Dr. Krippin in I Am Legend's opening lays out the origins of the Krippin virus. Dr. Krippin had genetically edited the measles virus in order to change its effects from damaging to beneficial, spreading a cure for cancer through the bodies of patients. The idea is similar in basic principal to how vaccines work: taking something that would otherwise be harmful to people and turning it into a way to save people. Krippin compares the process of creating her virus to replacing a dangerous driver behind the wheel of a powerful car with a cop.
Initially it seems that the virus is a success, with more than 10,000 cancer patients cured. However, it then begins to have much more lethal effects and mutates from a contact-borne virus to an airborne one. 90% of people who are infected with the virus die. 1% (about 12 million people on the planet) are immune to the virus. The remaining people transform into Darkseekers - pale-skinned creatures with a savage lust for flesh, enhanced strength and speed, and no apparent higher brain function. The only safe haven from the Darkseekers is daylight, since they experience severe reactions to UV light.
The long-term effects of the Krippin virus vary depending on which version of I Am Legend you're watching. In the theatrical cut ending (which was created after test audiences reacted poorly to the original ending), Robert Neville successfully finds a cure for the disease and Anna (Alice Braga) brings it to a survivors camp and hands it over to military officers. In the alternate ending, Robert realizes that the Darkseekers invading his building are led by an alpha who only wants one thing: the return of his mate, a female whom Robert has been experimenting on in an attempt to find a cure. This ending reveals that the Darkseekers are in fact still capable of feelings like love and attachment, and are able to show restraint from killing.
Why I Am Legend's Darkseekers Aren't Zombies
Though the Darkseekers might seem like zombies - with their mindless violence, lust for flesh, and contagious bites - the key difference between Darkseekers and zombies is that Darkseekers are not dead. In fact, they have extremely high body temperatures and extremely high heartrates, which go hand-in-hand with their aggression. Also unlike zombies, non-immune people do not need to be bitten by a Darkseeker in order to become infected. Though that's how the virus was originally spread, it later became airborne, leaving anyone who was not immune at risk of dying or becoming a Darkseeker merely by being in the same general vicinity as other infected people.
Although they may not fit the technical modern definition of a zombie, I Am Legend's Darkseekers do often get put into the same broad category of zombies. Whether or not the word strictly applies is a debate that has also been had about 28 Days Later, which is widely regarded as a zombie movie even though the zombies in it, like the Darkseekers, are still-living people who have been infected with a disease that causes extreme rage.
How I Am Legend's Darkseekers Are Different In The Book
In the book version of I Am Legend the monsters are more straightforward than in the movie: they're vampires. That label is used explicitly and their condition comes with a lot of the usual vampire traits, including blood-sucking, a hatred of garlic, and the requirement of a wooden stake through the heart to kill them. Unlike the Darkseekers, the vampires in Matheson's novel are nearly indistinguishable from humans and are still able to talk. At night, instead of hearing screams and howls, Robert is forced to listen to his vampire neighbor yelling at him to come out of the house. The novel's vampires are also cannibalistic, preying on their own kind when they cannot find a human to consume.
The origins of the vampires in Matheson's I Am Legend are also slightly different than those of the Darkseekers in the movie. In the book, Robert discovers that the source of the disease that turns people into vampires is a germ carried by dust storms and mosquitoes that plagued the world in the wake of a recent war's bombings. The ending of the book is closer to the alternate cut of I Am Legend, with Robert discovering that some of the infected have been able to resist becoming undead due to mutations in the bacteria, and have formed a new society. Just as Smith's Robert Neville does in the film's original ending, the book version of the character realizes that he has become a monster in the eyes of this new species. The final words of the book are "I am legend," as a dying Robert muses that one day he will become nothing more than a frightening superstition, just like vampires once were.