According to translated reports, Yoochun has been diagnosed with shingles. Shingles (varicella zoster virus) is a rash most commonly affecting the torso area.
A representative is quoted as saying:
"Yoochun was diagnosed with shingles last week while filming for `Miss Ripley`. His face is fine, but he has rashes on his chest and back. Due to his busy filming schedule this past week, he has been suffering from overwork and stress, which led to a weakened immune system and his illness.
"It would be best for him to get some rest in a hospital, but he’s receiving treatment in between schedules that we could not avoid"