"Epidemiology" is the sixth episode of the second season of the American comedy television series Community, and the 31st episode of the series overall. It originally premiered in the United States on NBC on October 28, 2010 as a special Halloween-themed episode. In the episode, the dean throws a Halloween themed party on campus. During the party, a few of the partygoers become sick from eating a hazardous substance that the dean mistook for taco meat bought at an army surplus store. The sickness causes those affected to turn into a violent, zombie-like state that can be passed on through bites. As the study group try to escape, the dean (under orders from U.S. Army Special Operations) locks the doors, trapping them inside with the infected.

The episode was written by Karey Dornetto and directed by Anthony Hemingway, and also features the voice of George Takei. The episode's plot is an homage to various zombie and horror movies, with the infected students mimicking stereotypical zombies and various tropes of the genre. The episode received overwhelmingly positive reviews and was watched by 5.6 million viewers on its original airing.

Outside the Greendale Community College library, Jeff (Joel McHale), dressed as David Beckham, greets Pierce (Chevy Chase), dressed as Star Trek's Captain Kirk for the annual Greendale Halloween party. The pair walk inside, where all the songs being played are by the 1970's pop group ABBA and Pelton's voice notes to himself, as they are using his iPod for the music. When Pierce asks Dean Pelton (Jim Rash), dressed as Lady Gaga, where he got the food, Jeff reveals it is old military rations bought at a surplus store. Pierce becomes ill, beginning to sweat and becoming disoriented. Annie (Alison Brie) finds Rich (Greg Cromer), the doctor from the beginner pottery class, and they discover that Pierce is running a temperature of 102°F. While Troy (Donald Glover) and Abed (Danny Pudi) attempt to court women with their Aliens-inspired costumes, other partygoers begin contracting the mysterious sickness. Although Rich initially assumes it is food poisoning from the rations, Pierce bites Star-Burns (Dino Stamatopoulos) on the arm.

The main characters (except Pierce) continue to enjoy the party, despite noticing that many of those attending the event appeared to be "high on drugs." Troy, who has by this time changed out of his science-fiction style costume and opted to dress as a "sexy Dracula," calls Abed a nerd, hurting his feelings. Meanwhile, at his office, Dean Pelton calls the surplus store to complain. When he reads a code on a biohazard-labeled package he mistook for taco meat, he is transferred to an army special operations officer, who orders him to quarantine those inside the library and wait until the army arrives in six hours.

At the party, Leonard (Richard Erdman) savagely bites a woman, and Troy and Abed believe that they are in the midst of a zombie attack. As the unaffected partygoers try to escape through the library's front door, the dean locks it from the outside and seals them in with the "zombies". The remaining survivors barricade themselves in the group study room.

Rich explains that the "zombies", who are actually still living people, are suffering from a rabies-like illness and will suffer brain damage and death within six hours. Annie and Rich suggest lowering the building temperature to lower the "zombies'" fever and kill the virus. However, the thermostat is outside with the "zombies". Rich emphasizes that everyone disclose whether they were bitten, even though he and Britta have covered up their bites. When they begin exhibiting symptoms, Chang (Ken Jeong) throws an ice skate at them to knock them out but ends up breaking a window, allowing the "zombies" to surge in and grab Annie.

The group flees the study room and splits up. Shirley (Yvette Nicole Brown) and Chang escape to a nearby bathroom, where they share a passionate moment and have sex. Jeff, Troy, and Abed get to the library's basement, where they spot a window. Jeff, not wanting his expensive suit to get dirty by going through the window, opens a door instead, letting in the "zombies" from the other side and getting overpowered. Abed encourages Troy to "be the first black man to make it to the end" and sacrifices himself, allowing Troy to escape through the window.

Once outside, Troy forcibly takes the keys from Dean Pelton and reenters the building to get to the thermostat. Abed, Britta, and Shirley bite Troy, but he is able to reach the thermostat before being affected. After the air conditioning comes on, everyone is healed. The army arrives to "disinfect" the building and ensure that the dean is the "only witness." Upon discovering those inside are uninfected, they opt to erase everyone's memories. In the aftermath, the party-goers assume the drinks were roofied. George Takei ends the episode with a voice over, and recording a voicemail greeting for someone named Kevin.