Question: A non-religious friend says he does not understand why he contracted a sickness, while a religious friend says she does not understand why the deity she has faith in allowed her sickness to occur. Why do even religious people get sudden and sometimes terrible sicknesses?
Answer: No religion promises 100% freedom from sickness and even death upon faith without adequate spiritual practice… or many in this world will never be sick and even live forever. This is while religions do offer various perspectives on suffering, as accepted by their adherents.
Why should the ‘innocent or good’ be ‘punished or tested’? Beyond medical ’causes’, on the question of ‘Why me instead of someone else?’, the Buddhist answer is that sicknesses are neither punishments nor tests from any deity or demon; but from personal past and/or present negative karma bearing fruit.
Thus, there is no mysterious cause for sicknesses, while acceptance, responsibility, repentance to make amends and merit-making is expected for possible healing. (Complete and lasting healing is only in Pure Land or when liberated.) However, a common karmic origin for sicknesses is having caused sentient being(s) suffering, in this and/or past lives, through greed, hatred and delusion.
As oneself is a sentient being too, these three poisons can also be expressed upon oneself to create sicknesses, such as by continual making of unkind and unwise choices of food and drink for example, that medically cause sicknesses. Medical causes of sicknesses are thus connected to karmic causes. Of course, there are other karmic causes too.
Some Buddhists would perceive their sicknesses to be ‘self-punishment’, but in terms of karmic retribution; not caused by any higher being, even if conditioned through others, as human and/or non-human karmic creditors. They can also see their experiences as personally deserved ‘tests’ to practise patience, empathy for others also sick and other related virtues.