Mahler was so upset by other composers who died after composing their ninth symphonies, that he refused to label his ninth symphony as a ninth symphony instead calling it -Das Lied von der Erde. But as we all know, that sort of human hubris always fails and he contracted pneumonia, I think -if it wasn't pneumonia it was something else that proved extremely unhealthy, while he was writing his 10th symphony and exited earth.