Jun Mung-gyu remembers the throbbing pain in his head and shoulder aches from spending as many as 15 hours a day hunched over a computer keyboard battling his online foes. For others, the addiction has become all-consuming, raising concerns about the health of the millions of gamers in the world’s most wired country. The habit has even been deadly: In August, a 28-year-old man died after nearly 50 straight hours of playing online computer games.