Contamination Situation

In which a main character is exposed to or infected with something very bad, usually potentially fatal and the exposure/infection is a main focus of the episode. In television, unless a character experiences a near-death revelation that completely changes them or dies, these episodes are typically not wham episodes, but are still usually a big deal, often airing during sweeps.

The infection usually advances Character Development by causing the character to rethink their life. In television, you can also usually expect a good moment or two from the golden couple.

In television, this is most prevelant on crime and medical dramas, not surprisingly. The exposure/infection usually, but not always, happens to a leading character as opposed to a supporting character. A common exception occurs when a lead character's love interest is a supporting character.

There are also countless films based on this premise.

All Spoilers will be unmarked ahead. Beware.

Examples of Contamination Situation include:

Live Action TV

House had three: "Euphoria" in season two, where Foreman is infected from a patient, "Airborne" in season three, where House and Cuddy are on a plane with someone who is suspected to have meningitis, and Cuddy thinks she has it, and "A Pox on Our House" in season seven, where House is exposed to an contagious patient.
Bones had one in season one. The entire team is exposed to an infectious body.
Fringe did theirs in season two. Peter contracts a pathogen at a crime scene.
- In an episode in season one, Charlie becomes an incubator for a supernatural creature.
On 24, after Jack contracts a pathogen from a bio-weapon, the entire second half of the seventh season is a string of contamination episodes.
The penultimate episode of the second season of NCIS centered around Tony and Kate's exposure to the pneumonic plague, which was sent to the team in an envelope.
In the late season four episode "Amplification" of Criminal Minds, Reid is exposed to anthrax by a serial killer.
In season four on Psych, Juliet is exposed to a virus during an investigation.
In an episode of Sliders, one of Quinn's friends gets the plague.

Film

In Contagion, Matt Damon’s character is infected with a virus, but is immune to it. Later on, one of the main characters has been infected with a virus and uses this to test the cure. Another main character dies from it.
In The Last Man On Earth, Vincent Price is the only person left and is immune to the virus turning people into vampires, but does have the virus.
In I Am Legend, the dog is infected with a virus that turns him into one of the mutants. Robert Neville has to kill his dog
In 28 Days Later, one of the main characters’ friends is infected, and she must kill him.
In The Cassandra Crossing, one of the main characters, a doctor, is infected with a virus and is considered to be an engineered biological weapon.
In Airplane! the mysterious sickness gets both the pilot and copilot

Western Animation

In Futurama, Fry has a dormant 20th C. strain of the common cold, which had been erradicated centuries earlier. It leads to the entire island of Manhattan being quarrantined and tossed toward the sun for good measure.