why did Henri Matisse fall ill?

Answer 1

He had surgery for intestinal problems, and that led to a long recuperation earlier in his life. He makes reference to his work and wanting to give it a feeling of Spring, to hide the demands that his arts placed upon him, and the subsequent impact on the quality of his life. There are no other references to the cause of his death.

Answer 2

According to his Wikipedia article he had cancer:

http://en.wikipedia.org/wiki/Henri_Matisse

but he died 13 years after his surgery for it, so I don't know if that's what offed him...

Answer 3

Although he had medical problems as he got older, including cancer, he probaly simply died of old age.

He was in his mid-80s when he died. That's old age, my friend.