This post is xex onlinepart of Mashable's You're Old Week. Break through the haze of nostalgia with us and see what holds up, what disappoints, and what got better with time.

Asylums for the psychotic emerged in the 1800s, when it was deemed immoral to simply throw insane people into jails. And among the most serious afflictions in these wards was nostalgia.

"It was once a medical disease that could end fatally," Edward Shorter, a professor of the history of medicine and psychiatry at the University of Toronto, said in an interview.

Today, nostalgia certainly isn't viewed as anything even approximating mental illness. It's thought of as an emotion, a mild, wistful longing for the past that is, in fact, good for you. But in the 1800s, among halls of the hysterical, delusional, and unintelligible, there were those suffering from a homesickness so extreme, it drove them mad.

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"People could leave home and become servants and then be overcome with homesickness," said Shorter. "Three weeks later, they’d be dead."

There's no doubt the affliction, stoked by a forced displacement, could progress to death, said Thomas Dodman, a historian at Columbia University and author of What Nostalgia Was: War, Empire, and the Time of a Deadly Emotion, in an interview.

"It's a serious thing, said Dodman. "It's not some sort of pre-scientific, medical joke."

In 19th-century France, medical academics tried in earnest to figure out what exactly nostalgia was. There were around 80 medical dissertations on the subject. "It was the second most studied [mental] disease, after hysteria," said Dodman.

Army doctors took it quite seriously, especially in cases of isolated soldiers, who had traveled into foreign, hostile lands. "Doctorswere very attentive to the fact that soldiers missed home," said Dodman. "They felt very alienated in the army."

While there's little question the disease could be fatal, it's more debatable how nostalgia actually killed those afflicted. Dodman argues that for two centuries, nostalgia manifested itself as something we might understand today as post-traumatic stress disorder, or PTSD.

"[Soldiers] grew so miserable and apathetic that they stopped eating and showed signs of clinical depression, and because of lack of hygiene and medicine, they contracted diseases that killed them off," he said. "Nostalgia is more often a predisposing cause to their death – but the doctors definitely thought of it as a potentially fatal disease."

Shorter, meanwhile, contends that we can still find the exact psychosis today, though under a different name: "lethal catatonia." After experiencing "extreme psychotic excitement," the afflicted fall into a state of extreme exhaustion and become feverish. At worst, they die.

"It wasn’t as common as the common cold, but it certainly did happen," said Shorter.

Although Dodman believes the Union army during the U.S. Civil war grossly underestimated the number of nostalgia afflictions, he found records showing that the Union army suffered 5,213 cases among white soldiers alone. Fifty-eight of these were fatal.

But the medical term nostalgia, originally conceived in 1688 by a young medical student, fell out of use at the turn of the 20th century, said Shorter. It became jumbled in with the illness "schizophrenia," and lost its status as a distinct, independent illness.

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"It’s obviously been forgotten about today," he said.

Instead, it's been replaced.

Nostalgia turns good

Today, the pleasant nostalgia we all know well is so widespread, scientists wanted to understand if it actually served a psychological benefit. And after a considerable amount of research, it seems that it almost certainly does.

"This wistful, sentimental love and fondness of the past serves a number of psychological functions," Tim Wildschut, an associate professor of psychology at the University of Southampton who has researched nostalgia for years, in large part with the university's Nostalgia Group, said in an interview.

"It reminds us of experiences, and imbues life with meaning," Wildschut said. It brings past or dead individuals to life, "so you feeling connected and not alone," he said, and it creates a strong sense of self, that "you are more or less the same person across time."

Wildschut has even studied how Syrian refugees in Saudia Arabia, who fled war and terror, are affected by the emotion. "By and large, nostalgia was also beneficial for them," he said, but noted they were still less optimistic about the future (and understandably so) than Western research subjects.

But Wildschut emphasizes that he didn't dive into nostalgia with an agenda to declare a novel psychological cure -- in an age where snake oil, sometimes in the form of healing crystals, sometimes in form of motivational gurus, is common.

"Our goal was not to find some sort of new-agey 'quick, get rich scheme' for your path to happiness," he said. "We just wanted to understand the emotion."

Although "nostalgia" was not associated with this healthy, wistful longing in the Western world until the 1900s, other cultures, like those in Ethiopia, China, and Japan, already knew about it, and had discrete words for it.

In the U.S. and other Western countries, it seems we couldn't exactly put our finger on this poignant feeling, even though we knew it was there.

"It seems there was an emotion in search of a name," said Wildschut.

A similarity between the deadly disease, and the positive emotion?

There's no question the two phenomena are hugely distinct.

"They’re talking about a mild longing for the past -- kind of like nostalgia for The Beatles," said Shorter, referencing Wildschut's research.

But even so, there might be a parallel between the two.

In the 1800s, treatment for extreme homesickness, and the psychosis that resulted, was apparently straightforward.

"They had no idea how to prevent it other than letting these people go home," said Shorter.

And today, experiencing a mild longing for the past may be like escaping to home or other better times -- especially during darker periods. People experience nostalgia when experiencing threatening circumstances, like the cold or violent storms, said Wildschut.

"This suggests nostalgia might be an eloquent method for searching out a state of being as if it were actually occurring, and escaping the unpleasant circumstance you find yourself in," said Wildschut.

"It's through nostalgia that you can simulate being in different circumstances than you actually are," he said. "If only briefly."

We likely did the same in the 1800s -- though we lacked such a pleasing word for it. Centuries later, we're mostly the same people; sometimes we long for things in healthy ways -- but there are apparently dark extremes.

"We are creatures that long for things we don’t have," said Dodman.

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