Beth Emhoff has a layover in Chicago on the way home from a business trip to Hong Kong. She uses the time to see a former love before heading home to Minneapolis, where her husband, Mitch, and two children live. Two days later, she experiences a seizure and dies at the hospital from an unknown condition. Soon after, Mitch's stepson, Clark, dies in a similar way. Mitch finds out that he is immune to whatever the disease is, and returns home to his teen daughter, Jory.
Representatives from the Department of Homeland Security convene in Atlanta with Dr. Ellis Cheever of the Centers for Disease Control and Prevention. Cheever suspects that the disease is a bioweapon released in time for Thanksgiving when people spend time together the most; the weaponized disease can do the most damage at this time. Cheever sends Dr. Erin Mears, who works for the Epidemic Intelligence Service, to Minneapolis to speak to Mitch Emhoff, because Beth has been identified as patient zero. After traveling to Minneapolis, Mears also becomes infected and dies.
Dr. Ally Hextall has been studying the virus. She finds that it is a mixture of genetic material from viruses that typically infect pigs and bats. Another doctor, Dr. Ian Sussman, defies instructions to destroy all of the sample cells, so that he is able to use samples to grow the virus and help devise a vaccine. At this time, it is also determined that the virus spreads by contact with inanimate objects.
Alan Krumwiede is a freelance reporter in San Francisco who thinks that the virus is a government conspiracy. He posts a lot of videos and in one he states that he has cured himself of the virus using homeopathic medicine. He cites forsythia as the main curative plant. People flock to pharmacies demanding the forsythia extract that he has been using. However, we later learn that Krumwiede is faking this miracle cure in order to boost the sales of the forsythia extract. He is arrested and charged with securities fraud and conspiracy.
Dr. Cheever is under investigation because he told his family and friends to get out of Chicago before it was quarantined. Hextall, meanwhile, is confident that she has developed a vaccine and is anxious to test it out. She does not want to wait for the informed consent of patients who already have the virus and so she vaccinates herself and then visits her father who has the virus. She does not contract MEV-1 and so the vaccine is announced a success. Vaccines are issued via lottery, as the death toll worldwide rises to twenty-six million.
World Health Organization epidemiologist Dr. Leonora Orantes is kidnapped while doing research in Hong Kong, a maneuver orchestrated by a Chinese official named Sun Feng, who wants to use her as a trade in order to get the vaccine for his small village. Vaccines are provided and Orantes is released, but on her way out of town, Orantes learns that the vaccines provided to the village were placebos.
The cause of the virus is revealed; before Beth was infected, a bulldozer on a construction site knocked a tree down, disturbing the bats living in it. One bat flew over a pig sty, dropping a piece of banana. One of the pigs in the sty ate it, and when the pigs were slaughtered, they passed the virus on to a local chef, who then shook hands with Beth when they meet at a local casino.
