A person can fulfill his purpose in life equally as well in illness as in health.
If we meditate upon the existence of life after death, then all illnesses would seem to be taking us closer to the movement from one life to another, a transfer more desirable than undesirable. During these pains we can understand and explain for ourselves what will happen to us, and prepare for the new state of our next existence.
Usually people think that it is possible to serve God and humanity only when you are healthy. This is not true. Very often it is to the contrary. Christ served God and all mankind when he suffered on the cross, and even then he forgave those who killed him. Every person can do likewise. You cannot say which state is better, a healthy or ill state, in the service of God and humankind.
For yourself, and for yourself only, it is necessary to have health and power. But to serve God it is not necessary, and sometimes even the opposite is true.
Very often, when I have dealt with terminally ill people, I have learned that the most important thing is not hiding the approaching death from the patient, but on the contrary, explaining to him his own divine, spiritual nature, which grows in him and which cannot be destroyed by death.