Instant Illness

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Mother always told you, "Put your jacket on, you'll catch pneumonia!"

In Real Life, exposure to the cold weakens your immune system, and damages the cilia in your respiratory system that sweep the microbes away from your lungs. But you don't always get sick in bad weather, especially if you're a healthy twentysomething. Moreover, in real life infections have an incubation period. Symptoms usually don't appear right away.

In TV land, a man endures the elements for just a few hours for a woman and suddenly he's got a fever so bad he collapses. The obvious solution is Intimate Healing, because it always happens at night and hospitals are just too inconvenient or far away. This sometimes comes from Caught in the Rain, if the man lends his jacket to the woman until they find shelter. The man sometimes wakes up in the Apply Head Directly to Lap position. Sometimes leads to the Florence Nightingale Effect. While the man's unconscious, he can talk in his sleep. The woman might change her clothes in front of him too, after all, he's not going to see anything. Unless he wakes up right then, of course.

This tends to happen very early in a relationship, shortly after the couple meets for the first time.

Examples of Instant Illness include:

Anime and Manga

In the Ah! My Goddess OAV, after Keiichi brings an unconscious Belldandy to the abandoned temple, he collapses from a fever and wakes up with his head in her lap.
In Ai Yori Aoshi, Kaoru walks through the rain to get Aoi, who's just gotten lost for the second time on the Tokyo train system. After they get back to his apartment, he collapses from a fever and Aoi has to save him with some very Intimate Healing.
In Yuu Watase's manga Imadoki!, early in their acquaintance, Kugyou finds Tanpopo planting flowers in the rain, and she suddenly collapses with a fever. She hates hospitals, so he takes her back to her apartment instead, and has to take care of her—including changing her clothes while she's unconscious.
This happens a few times in Hana Yori Dango but the most ridiculous example is when Tsukushi falls into a swimming pool in a warm, tropical setting and instantly gets a bad fever. This prevents her from getting physical with Domyoji when they are finally alone together.
In the fourth episode of Seitokai Yakuindomo, Tsuda seemed perfectly healthy until the moment he steps out from under an umbrella. Within seconds he sneezes and is sick for the next episode.

Literature

In Harry Potter, the Weasley twins develop a range of sweets that make one ill and test them on fellow students. Hermione is unamused and shuts them down.
- Tries to, anyway.
The Chalet School books are pretty bad for this, probably because of Values Dissonance and girls being considered to be delicate, as well as the school's large Ill Girl contingent. Any exposure to chills, rain, drafts, mud or cold water is fairly certain to leave the victim in the grip of a life-threatening disease if they're not immediately put into a hot bath and then into bed with two hot water bottles. In one book, Jo spends three days unconscious and is bedridden for over a week after standing for literally a few minutes by an open door on a snowy day.
This is artificially induced as part of the hero's alibi in Andre Norton's At Swords' Points. He's handed a handkerchief and told to inhale sharply three or four times. For the next several days, he'll then have all the symptoms of a bad head cold, which will help explain why he wasn't out and about and didn't know the police wanted to ask him questions.

Western Animation

Invoked in The Adventures of Jimmy Neutron, when Jimmy invents a "sick patch" that make the wearer sick when applied, which the kids then use to stay home from school.