Spirit knows not of disease
Those who talk of unfairness are still thinking in terms of bodies, of a world of matter, and not of an infinite life. The spirit does not suffer from disease. The spirit is not crippled or mis-shapen or bent. The spirit is not suffering from any hereditary traits or any of the acquired characteristics of the parents. These do not change the individual, although they do affect the body through which the spirit manifests on earth.

Whilst you can quite possibly argue that, from the earthly point of view, looking at life soley from a material standpoint, the one who is born into a diseased body has a much worse time physically than the one born into a healthy body, those opinions do not hold in regard to the spirit which is behind the body. You will not automatically be poorer in spirit because your body is diseased, and richer in spirit because your body is healthier. Indeed, it can be argued that your spirit will be richer because you will have learned the many lessons of pain and suffering which are all part of the equipment of the spirit in its essential evolution.