Mental illness common in Japanese visiting Paris: symptoms include physical malaise, paranoia, feelings of persecution, sometimes megalomania and hallucinations.

Why Paris? Why Japan?  The answer lies in the special emphasis on Paris as the ideal foreign city in Japanese media.

Japanese are sold on an idealized version of the city: Parisians, they are told, are elegant and refined-- the air is always balmy, lovers walk hand in hand by the Seine on quaint, well-kept streets, artists and mimes ply their trade in every park, if not every street corner, French men, whether in an opulent salon or as waitstaff are invariably charming, French women are stick-thin, always dressed in high-end fashion (and always available), and, most of all, they're just like the Japanese, deep down.

    If this seems incredibly oversensitive and naive, just know that to Americans, London underwent the same kind of Disneyfication fifty years ago: we were pretty much sold on the idea that English people were much like they were in musicals and classic movies -- the men, impeccably well-dressed and mannered, children completely well-behaved and better-educated (they knew Latin! imagine!),  and the women, horsey and tweedy. If they were, just maybe, a little sexless and overly proper, it was our horrible American manners and vulgar habits of speaking showing through. (Of course, there was something called the working class, but they discreetly kept out of the way, unless they were someone picturesque, like a chimney sweep or a maid.)
    Nowadays, we have a somewhat more enlightened view of things: fifty years of rock and roll, British TV, and the fall of the house of Windsor to the status of a soap opera has somewhat demystified ordinary British life. We know that British children don't always act like Wednesday and Puggsley Addams, British women are capable of designing, sewing and wearing clothing that makes them drop dead gorgeous (and not a few of them can cook, too!), British men come in all shapes, sizes and accents, and that the upper class, the educated class and professional actors are far from congruent. Also, we've kind of figured out that London is just another modern city, and people are likely to be doing other things than entertain the tourists.

    Not so the Japanese. For them "Paris" is not only the City of Lights, but a kind of hyper-Kyoto of utmost beauty, elegance and comfort everywhere. Finding out, however, that a) most French people don't know any Japanese, b) Paris is noisy, dirty and sometimes just plain ordinary, c) shopkeepers and waiters can and do ignore people who don't explicitly ask for attention and often give snappy backtalk as a way to show their equality with the patron and d) no, you can't pretend you're back in Japan, physically, mentally or spiritually, causes at least twenty visitors a year to completely go around the bend. One fellow took to standing nude in his hotel room, announcing to the help that he was "the Sun King" and demanded that they dress and feed him, others simply feel invisible, depressed and/or slightly nauseous. French humor,  and peoples' casual attitude in general towards each other makes many Japanese feel as if they're being singled out for punishment. Jet lag, unfamiliar food and the compulsion to pack each precious moment ("you're in Paris!") with activity also is a factor.

Treatment is given according to case, but most sufferers get better after getting to talk to other Japanese, a few days rest, and, in the worst cases, a supervised trip home. Few, if any, show lasting symptoms.