"According to Johnny’s Entertainment, Aiba felt sharp chest pains during the filming of “Arashi ni Shiyagare“. After the filming ended, he immediately went to the hospital for examination, and was hospitalized. On the 29th, he was diagnosed with spontaneous pneumothorax. Aiba will be hospitalized for about a week, then will briefly rest for his health."
A spontaneous pneumothorax is where your lung collapses due to air being in the chest cavity outside of it. My boyfriend had this happen to him when he was 16. The doctors told him it sometimes happens to tall skinny dudes for no apparent reason. He said it hurt a lot when it first happened because all the air outside his lungs was putting pressure on his heart, and it hurt when they took out the chest tube, but other than that, it wasn't so bad.
I hope Aiba-san feels better soon! I can't believe he finished filming and then went to the hospital.