Part 1

At a Chicago Bears game, members of Firehouse 51, the Intelligence Unit, and Chicago Med are tailgating. After Matthew Casey notices a guy collapse in the middle of the festivities, they head over to help and find the guy with a flesh-eating infection on his legs repeating the letters "BRT." The guy is taken to Chicago Med where they identify the infection as necrotizing fasciitis. Ambulance 61 gets dispatched to a house where they find two people dead inside from the same infection. Jay Halstead identifies the original victim as a student at a local university. Cruz reveals to Mouch and Brett that he plans on proposing to Chloe. Burgess responds to a call at a gas station where a woman and her child are both infected. They both shortly die later at Chicago Med. Boden begins dealing with the mayor's office when the house is assigned to parade duties. Firehouse 51 responds to a fire at Central Chicago University (CCU), where a large chemical fire has broken out. While in the basement rescuing trapped people Severide notices a panel on the wall with the same "BRT" initials. The discovery allows intelligence to begin putting together a connection between the infection and the university. After shift Chloe breaks up with Cruz. Intelligence discovers that the original victim was a lab assistant at CCU and had previously been threatening professors with an apocalyptic-type event happening in Chicago. Burgess and Upton go to Chicago Med to question him; however, when they arrive, they are told he died. The Office of Fire Investigation determines that the fire at CCU was deliberately set. Jay Halstead finds out that another lab assistant at CCU, Veronica Song, was supposed to be in the lab at the time of the fire but didn't show up. Upton arrives at Song's house to question her; however, upon entering Song collapses from infection. They are both brought to Chicago Med where Song is rushed into surgery.

Part 2

The press begins questioning Goodwin about the infection. Upton gets examined for potential infection after finding Song's blood on her; Song later flatlines in surgery and is unable to be revived. The Center for Disease Control (CDC) arrives to begin tracking down the source of the infection. Ambulance 61 along with two other ambulance companies get dispatched to the same apartment complex where they find five infected patients. The CDC along with Chicago Med, Fire, and P.D., evacuate the apartment building in hopes to finding more leads. P.D. narrows down all sources of the infection to either CCU or the apartment building and identifies it's being spread via a bug sprayer. The CDC thoroughly inspects the apartment building and finds a bug sprayer being burned in the furnace. Voight and Upton begin interviewing tenants of the apartment building who all collectively describe a man who appeared to be an exterminator. CCU students reveal that they were working with the infection hoping to make a treatment for it although its progress was destroyed in the fire. One apartment tenant mentions a blue pickup truck with a grey circle which intelligence begins searching for. When the apartment tenants get impatient they all escape quarantine but most are later found. Upton's test comes back clear and she is released. Intelligence tracks down a blue pickup in the vicinity of the apartment building and the tag plate comes back to a car rental company. CCU students along with Will Halstead continue looking for a treatment and begin making progress. Burgess and Ruzek question the car rental employee and get security footage, the renter is positively identified as an employee of the CCU lab. Meanwhile, Firehouse 51 is dispatched to a public disturbance call. Will Halstead walks in on a lab assistant, David Seldon, destroying microbiology samples, the two engage in a fight but Seldon ultimately wins by hitting Halstead in the head with medical equipment, knocking him unconscious, and runs away.

Part 3

Voight and Jay Halstead arrive at Chicago Med looking for Seldon who had been identified as the truck rentee but he is nowhere to be found. At the public disturbance call, Firehouse 51 and P.D. have trouble maintaining order when civilians believe they know who caused the outbreak and want to take matters into their own hands. Chicago Med is put on lock down as they begin searching for Seldon. Jay walks by the security guard and tells him Voight is looking for him. Both Voight and the security guard walk down the hallway of Chicago Med feverishly. The security guard explains that a ”Code Silver” is the mandated code for a shooter in the hospital. Hospital security cameras reveal he already escaped so they begin broadening their search. Jay Halstead and Upton find a house they suspect Seldon to be in although all they find is his ex-wife. Intelligence questions the ex-wife who tells them Seldon isn't done yet. They also end up on a wild goose chase when they get many leads in different areas around the city from civilians trying to help and Seldon setting up false leads. The team finds his lab with many medical experimentation tools and a map with an area circled that includes a parade route. Firehouse 51 arrives to assist with the parade only to find the route to have no spectators due to widespread fear from the infection. Intelligence finds out that BRT had previously been funding Seldon experiments but when funding was unexpectedly cut he snapped and started planning to release the infection. Further investigation leads them to believe that Seldon is planning on releasing the infection at BRT headquarters where a board meeting is taking place. The CDC and Firehouse 51 are called to help evacuate the building. Meanwhile, the team at Med working on a treatment finds a proper antibiotic that attacks and reduces the infection. Atwater, Jay Halstead, and Upton begin evacuating the building. In the boardroom Halstead finds the entire board taken hostage by Seldon and begins trying to deescalate the situation. Upton, with a sniper rifle on the adjacent building, creates a distraction which allows Voight and the rest of the team to enter. Seldon refuses to surrender and is shot dead by Voight. After wrapping up the case everyone begins settling down back into their own lives.

Main

Notable guests

Critical response

Vlada Gelman with TVLine said that the "crossover proved to be a monumental event for some of the Windy City’s finest."^[7]

Viewing figures

The three-part event aired from 8:00-11:00 pm EDT on October 16, 2019, on NBC.^[8] Chicago Fire switched timeslots with Chicago Med for the evening with Fire airing an hour earlier than usual.^[9] The first part of the crossover was watched live by 8.23 million viewers, the second part rose to 8.93 million, the final part dropped back down to 8.62 million.^[10] The high viewing figures led to the crossover being called the "dominant event of the night."^[11] Deadline Hollywood said "NBC’s three-part Chicago crossover titled “Infection” came on strong Wednesday night, giving a boost to all three franchises for NBC in the adults 18–49 demographic."^[12]