It has been announced that Aiba Masaki has been hospitalized. He was feeling pain in this chest at a taping and went to the hospital straight after wards. The ailment has been revealed to be a collapsed lung (spontaneous pneumothorax) which is an ailment he suffered from back in 2002.
Tokyohive article. (English)
Apparently it's one of those things that if you suffer from it once then there is an increased risk of it happening again. So hopefully Aiba will get some extra rest so he can recover well and not over strain himself when he gets back to work.