<P> An animated prequel, Seoul Station, also directed by Yeon, was released less than a month later . </P> <P> Seok - woo, a divorced fund manager, is a workaholic and absentee father to his young daughter Su - an . For her birthday the next day, she wishes for her father to take her to Busan to see her mother . They board the KTX 101 at Seoul Station . Others on the same train include the tough working - class husband Sang - hwa and his pregnant wife Seong - kyeong, a high school baseball team, the rich and egotistical COO Yong - suk, elderly sisters In - gil and Jon - gil, and a homeless man experiencing post-traumatic stress disorder . </P> <P> As the train departs, a convulsing young woman boards the train with a bite wound on her leg . She soon turns into a zombie and attacks a train attendant, who then turns into a zombie . The virus quickly spreads throughout the train . Baseball player Yong - guk, a girl named Jin - hee, who has a crush on him, and several passengers escape to another car . News broadcasts report zombie outbreaks (reported as riots) nationwide and Cheonan - Asan station was attacked by zombies so they passed it . After the train stops at Daejeon station, the surviving passengers find the city and station also overrun, and they hastily retreat back to the train, splitting up into different train cars in the chaos . The conductor restarts the train to head to Busan, where a quarantine zone has reportedly been established . </P> <P> Seok - woo, Sang - hwa and Yong - guk fight their way to where Su - an, In - gil, Seong - kyeong and the homeless man are hiding . Together, they struggle through the zombie horde to the front train car, where the other passengers are sheltered . At the instigation of Yon - suk, however, the passengers block the survivors from entering, fearing that they are infected . Sang - hwa and In - gil sacrifice themselves to give the others time to force open the door and enter the car . Yon - suk demands that the newcomers isolate themselves in the vestibule, and the others follow his lead . When Jong - gil deliberately opens the door to the zombies to be with her zombified sister In - gil due to Yon - suk and the other passengers not wanting the newcomers be with them in the train car, the zombies kill the rest of the passengers, leaving Seok - woo, Su - an, Seong - kyeong, Yong - guk, Jin - hee and the homeless man safe, as they are in the vestibule . Yon - suk and the train attendant escape the onslaught by hiding in the bathroom . </P>

What is the cause of zombies in train to busan
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