ANSWER: The disabling symptoms matter most, although diagnosis of a fatal or very serious disease can certainly be expected to have some impact on the disability decision. For instance, people with cancer do not automatically qualify for disability retirement unless they are presently suffering from the disabling symptoms of that disease. On the other hand, you don't have to suffer from any particularly serious disease, or even from any diagnosable disease in order to qualify for disability retirement. You only need to make the appropriate showing that you are suffering from disabling symptoms, even if the cause of the symptoms is unknown.
The rules are tough in not allowing a mere diagnosis to serve as a ground for disability retirement. But the rules are also lenient in permitting you to qualify for disability retirement on the basis of disabling symptoms which may appear to be minor or which effect you only on particular occasions. A person whose job requires travel by plane on occasion, but who is unable to fly on account of chronic back pain may well be eligible for disability retirement, even if an asymptomatic cancer patient is not eligible.