Also disposed to nostalgia were children sent to the countryside to nurse (who naturally missed their mothers), young men between 20 and 30, and women who left home to be domestic servants. Autumn was a particularly dangerous season, the falling leaves perhaps reminding marching soldiers of their impermanence and making them wonder why they were spending their limited time on this Earth bloodying their swords in distant lands instead of enjoying the comforts of home and hearth.
Aside from the nostalgia epidemic itself, there was also an outbreak of fake nostalgia among soldiers, who would pretend to miss their friends and family to get out of fighting. But the joke was on them, as "true" nostalgics would just retreat into themselves, without revealing why they were suffering, according to Michael S. Roth's Dying of the Past: Medical Studies of Nostalgia in Nineteenth-Century France.
Apparently, almost anything under the sun could cause nostalgia. A too lenient education, coming from the mountains, unfulfilled ambition, masturbation, eating unusual food, and love ("especially happy love," Roth's paper notes) could all bring on the disease. In the 18th and 19th centuries, some doctors were convinced nostalgia came from a "pathological bone" and searched for it to no avail.
Some of the symptoms victims presented with are fairly logical--melancholy, sure; loss of appetite, okay; suicide, upsetting but understandable. But many other symptoms that were gathered under the umbrella of nostalgia almost certainly had causes other than homesickness--malnutrition, brain inflammation, fever, and cardiac arrests among them. Some of the early symptoms, according to Dr. Albert Van Holler, were hearing voices and seeing ghosts of the people and places you missed, though whether these were hallucinations or just regular old dreams is unclear.
How to treat this primordial sludge of symptoms depends on the situation and, I guess, your perspective. For a little boy who missed his wet nurse, doctors brought her back and then slowly conditioned him to spend time away from her. The soldiers sometimes were treated with less patience. French doctor Jourdan Le Cointe thought nostalgia should be treated by "inciting pain and terror," as Svetlana Boym describes in her book The Future of Nostalgia.
Le Cointe cited the example of the Russian army's outbreak of nostalgia in 1733, on its way to Germany. The general told the troops that the first one to come down the nostalgic virus would be buried alive, and actually made good on his threat a couple times, which nipped that right in the bud.
When nostalgia finally made its way to the United States, after the Civil War, the "scare it out of them" tactic was replaced with "shame it out of them." American military doctor Theodore Calhoun thought nostalgia was something to be ashamed of, that those who suffered from it were unmanly, idle and weak-willed. He proposed curing it with a healthy dose of public ridicule and bullying. Maybe this is why most people don't feel nostalgic about middle school.