Task: Question Generation
Input (English):  In 1967, a pregnant woman is attacked by a vampire while giving birth. Doctors are able to save her baby, but the woman dies of infection.
Thirty-one years later, the child has become the vampire hunter Blade. He raids a rave club owned by the vampire Deacon Frost. Police take one of the vampires to the hospital, where he feeds on hematologist Karen Jenson and escapes. Blade takes Karen to a safe house where she is treated by his old friend Abraham Whistler. Whistler explains that he and Blade have been waging a secret war against vampires using weapons based on their elemental weaknesses, such as sunlight and silver. As Karen is now "marked" by the bite of a vampire, both he and Blade tell her to leave the city.
Meanwhile, at a meeting of vampire elders, Frost, the leader of a faction of younger vampires, is rebuked for trying to incite war between vampires and humans. As Frost and his kind were not born as vampires and are therefore not pure-bloods, they are considered socially inferior. In response, Frost has one of the elders executed and strips the others of their authority.
Upon returning to her apartment, Karen is attacked by a policeman, who is a "familiar", a human slave controlled by a vampire. Blade subdues the familiar and uses information from him to locate an archive of vampire history. Later, at the hideout, Blade injects himself with a special serum that prevents him from succumbing to his desire to drink blood, which would ultimately turn him into a vampire. However, the serum is beginning to lose its effectiveness due to overuse.
While experimenting with the anticoagulant EDTA as a possible replacement, Karen discovers that it explodes when combined with vampire blood. She manages to synthesize a vaccine that can cure the infected, but learns that it will not work on a human-vampire hybrid like Blade. Frost and his men attack the hideout, infect Whistler, and abduct Karen. When Blade returns, he helps Whistler commit suicide and arms himself with special syringes filled with EDTA.
When Blade attempts to rescue Karen from Frost's penthouse, he finds that his mother is one of Frost's followers. He is subdued and taken to the Temple of Eternal Night, where Frost plans to perform the summoning ritual for La Magra, the vampire blood god. Blade is drained of his blood, but Karen allows him to drink from her, triggering his vampirism. Frost completes the ritual and obtains the powers of La Magra. Blade kills all of Frost's minions, including his mother, and confronts him. During their fight, Blade injects Frost with all of the syringes, causing his body to explode.
Karen offers to help Blade cure himself, but he asks her to create a new serum instead. In a brief epilogue, Blade kills a group of Russian vampires.
Output (English): Why did Blade and Whistler tell Karen she she needed to leave the city?