Agency NH Media stated:

On the way to a scheduled event, Dongho experienced pains so he was taken to the emergency room. After tests, they found that he’s suffering from pleurisy (inflammation of the lining of the lungs and chest that leads to chest pain, usually sharp, when you breath or cough).

Dongho is currently acting as the spokesperson for the movie Don't Cry Mommy and is promoting U-KISS’ 7th album Stop Girl. Without Dongho, the rest of UKISS' members are continuing with their scheduled plans. Although Dongho's schedule and work is important, Dongho’s health comes first. We’ll be watching his progress after he receives treatment at the hospital and we will figure out whether or not he needs surgery.

U-Kiss' album is about to come out soon with only 6 members :C -sigh- I really to respect why hes leaving, but what would happen if he didnt have health problems? Would he still have stayed? He also expressed that he wants to live as a normal person rather than that of a celebrity...

I hope Dongho oppa gets better soon <3 It understand able that he wants his old life back, and he debuted when he was only 14 (15) and im sure he misses his old friends and going out without having fans and camars flashing and screaming all around him. I will always love Dongho for who he is, a idol or not I still love him <3

#GetBetterSoonDongho <3