Why people may be born disabled?

This is a similar question to one that has been deleted a while ago while I was in the process of answering, therefore I am posting this question in the purpose of helping others who may seek the same query! 1. Why is it that some of us are born deaf, blind or with any other disability or developed them through their lifetime? 2. Does this mean that the healthy have an advantage over the sick? 3. Are the sick rewarded for their patience during sickness?