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Mosquito capable of carrying harmful Japanese virus discovered in Seoul for first time this year
(koreajoongangdaily.joins.com)
Despite its name, Japanese Encephalitis is rarely found in japan either. it is most commonly found in south east Asia where it is very humid and hot(ter). Chances are the global warming will allow it to spread further north, but whether that was the cause this time, who knows.
Most Japanese children are vaccinated against japanese encephalitis so I'm going to assume Koreans are as well, but for those traveling, you might want to consider getting vaccinated before going. (Not sure about Korea, but It is pretty expensive for a vaccine in Japan for some reason, unless you're a toddler in which case it is free)