It’s an unidentifiable gastro intestinal illness, according to him. But he has to go to the hospital for an IV drip every couple of months due to it and excessive working with his condition.
He talks about it in his Biography, Jihaku:
This continued, and when I was ten years old, I suddenly collapsed. I had a violent pain in my stomach and intestines, and I couldn’t move.
After I was taken to the hospital, I was told that the cause was unknown. They told me that while that was the case, I probably had some sort of infectious disease.
So I was suddenly isolated. Isolated, in confinement, thrown into a hospital ward that was more a prison. I think that they put me in the pediatric ward because I was so young. Children who were heavily ill, had infectious diseases, or had terminal illnesses were in that ward. Being ten years old, that’s what I thought. All of them were in a cage, and were likely to go down the corridor at any time.
There’s also this blog excerpt:
When I was a child, my body was weak,
and I couldn’t help but be jealous of healthy people.
I couldn’t help but loathe the hospital.
When I accidentally found out during my long hospitalization
that I would only live to be 30,
then I fell into despair.
“Why just me?”
I lived thinking that.
I thought I was the most unlucky person in the world.
I was afraid of dying.
And he faints after every live:
Hope this helps.