Flu

The virus spread rapidly because it mutated into a highly aggressive H5N1 variant inside the shipping container, where poor conditions allowed it to become airborne and lethal. When the brothers took Monssai to a clinic, the contagion spread through staff and patients. The situation worsened when rats that fed on the incinerated bodies escaped into the city, spreading the mutated virus further.

The movie earlier shows the cramped, unsanitary conditions in the shipping container where the virus could mutate.

Hospital staff and the KCDC incinerated the shipping container in an attempt to destroy the source of the infection. However, rats that had fed on the contaminated remains escaped into the city before the burning was complete, allowing the virus to spread beyond the initial outbreak site.

Monssai, the sole survivor from the shipping container, chose to rescue Mi-reu from being hit by a car despite knowing his infection could accelerate transmission. This act suggests he wanted to prove his humanity and make amends for inadvertently bringing the virus to the city, even though he was already symptomatic.

After officials rejected a proposed direct antibody transfusion for patients in the isolation zone, In-hae secretly began a blood transfusion for her daughter Mi-reu. This was an attempt to save Mi-reu using her own antibodies, which later proved effective when Mi-reu began recovering from the virus.

Earlier, a proposed antibody transfusion for patients is rejected by officials, prompting In-hae to take matters into her own hands.

Byung-ki killed Monssai as a suicidal act of revenge for his brother Byung-woo's death. Byung-woo had fallen ill after capturing Monssai from the shipping container, and Byung-ki blamed Monssai for bringing the disease that killed his brother. The attack occurred during the chaos of the mob violence at the highway.

From the beginning, Byung-ki refuses to answer questions about the container and is desperate to save his brother, establishing his intense grief and blame toward Monssai.

The mob violence erupted when Gook-hwan, an infected agitator, led the group toward the highway in a desperate attempt to escape the quarantine. When passage was denied, he opened fire, triggering a fierce exchange between the rebels and soldiers. The situation was exacerbated by rumors that infected people were being killed and the government's broken promise to release the uninfected after 48 hours.

Earlier, pressure from WHO and political leaders forces the president to abandon the pledge to release uninfected people, directly fueling the anger.

Mi-reu recovered because In-hae secretly performed a blood transfusion using antibodies. Additionally, Mi-reu had been saved by Monssai earlier, and it's implied that her exposure may have triggered her immune response. By Day 4, Mi-reu makes a full recovery and her blood becomes valuable for vaccine development.

Monssai saves Mi-reu from being hit by a car, and In-hae later begins a secret transfusion for her daughter.

The quarantine became increasingly strict because the H5N1 variant could kill within 36 hours and spread rapidly through airborne transmission. The military enforced heavy restrictions, moving the population to a camp outside the Tancheon Sports Complex while confining infected people to a quarantine zone beneath the stadium. Violence erupted due to deteriorating living conditions, cut communications, rumors that infected were being killed, and the government's failure to keep its promise to release uninfected people after 48 hours.

The KCDC formally quarantines the city after determining the virus is a highly aggressive H5N1 variant that can kill within 36 hours.

The movie ends with Mi-reu making a full recovery after the secret transfusion. After In-hae is wounded protecting her daughter, Mi-reu shields her mother and pleads with the crowd, prompting the president to order soldiers to stand down and cancel an air strike. Mi-reu is then sent to Seoul to help in vaccine development, while medical teams are mobilized to Bundang, signaling a fragile step toward containment after the harrowing outbreak.

In-hae's car had tumbled into a mine shaft the day before the outbreak, causing her to lose crucial data about the virus. Her bag was later recovered by emergency response team members Oh Ji-goo and Bae Kyung-ub, who delivered it to her daughter Mi-reu after answering a call. This data loss initially hampered the Contagion Center's response to the emerging crisis.

Gook-hwan was an infected agitator who led the mob toward the highway in an attempt to escape the quarantine. When soldiers blocked their path, he opened fire, triggering the violent confrontation. His desperate bid to transfuse Mi-reu's blood (believing her immunity could save him) was thwarted when Kyung-ub intervened, and Gook-hwan was killed in the scuffle.