illustration (attribution, if any possible, is at the end of the article)
This might seem a preposterous question, or some sort of sacrilege, but Buddhism claims to talk about "reality as it is", not merely a fantasy…
The Buddha experienced back-aches (AN 9.4), had a fever (SN 46.16), his foot hurt by a fling rock splinter (Miln 1:136), and he felt the symptoms of dysentery during his last days (DN 16).
Some might be seen as karmic residue (after nirvāṇa, he no longer created new karma, but still faced the consequences of old karma ripening… until parinirvana).
However, the Saṁyutta commentary explicitly explains the Buddha's back-ache as arising from non-karmic causes, i.e. present conditions: « Why did it [his back] pain him? The Blessed One, who had devoted himself to the great exertion for six years [as an ascetic], had a great deal of bodily suffering. Later on, when he was very old, he had back trouble. That [backache] had no karmic cause. » (SA 3:52)
Mental vs. physical
The Roga sutta (AN 4.157) offers an important connected teaching: «
Monks, there are these two kinds of illnesses [diseases]. What are the two? Illness of the body and illness of the mind.
Bhikshus, there are to be seen beings who can claim to be physically healthy [illness-free] for a year… two years… three years… four years… five years… ten years… twenty years… thirty years… forty years… fifty years… who can claim to be healthy for a hundred years.
But bhikshus, hard to find are those beings who can claim to be mentally healthy for even a moment except for those [arhats] whose mental cankers are destroyed.
This has been the cause of much confusion, where buddhists start differentiating 'pain' from 'suffering'… and assert that the Buddha doesn't mentally 'suffer' from physical 'pain'.
There is some truth in this: anticipation of pain is itself unsatisfactory, regrets associated to pain are themselves unsatisfactory, etc., i.e. mental pain can be compounded with physical pain… and it is true that sentient beings beyond the illusion of 'self' and affected by neither 'lust' (of better circumstances) nor 'aversion' (of current circumstances) do not compound pains this way, do not create mental pain!
But this is not enough: not doubling the pain is good, but it isn't the same as not experiencing the 'primary' pain… The dualistic separation between 'pain' from 'suffering' is too superficial (or maybe it is still firmly stuck in an extreme associated to the conventional truth, totally ignorant of the ultimate truth).
Beyond pain and no-pain
'Pain' (as understood by the ordinary mind) is not experienced as such by arhats.
The phenomenon labeled 'pain' (by minds having preferences and wishes for the world to be different from what it is) is seen as part and parcel of the compassionate 'effort' arhats make to help all sentient beings.
Arhats don't fall prey of the fallacy of time, they understand causality without forcing a 'linear' model of time on it. When they 'invest' or produce some effort for the benefit of all, they understand that this might result in 'pain' as a 'consequence', but they can appropriate such 'consequence' as part of the 'investment' they make (thus they experience it as a 'cause' of the benefit created for all)! They appropriate this phenomenon and the others' suffering as part of the call for compassionate response, they produce the effort in order to respond, and the wholesome consequence is a benefit for all (including themselves: they might not need to produce such 'effort' again once the world is less dissatisfactory for all!).
Arhats embody patience and perseverance; they don't suffer from an 'effort', e.g. out of wishing the effort could be 'less' for the same result, or wishing such an effort wasn't required in the first place…
How do you interpret « this [backache] had no karmic cause » now? Without a karmic cause, it wasn't a karmic consequence; it was part of benefitting all beings, part of teaching (by example) the inadequacy of asceticism, part of the compassionate investment (with enthusiasm, rejoicing of the benefits reaped by all) and not part of the return…
As per Nāgārjuna's Mulamadhyamakakarika, causality is empty of essence: a 'consequence' can be experienced as a 'cause' (this is similar to the more classical "the consequence of one action is the cause of the next"), it's a set of labels, a narrative on top of the actual ineffable web of causality! 'Pain' isn't intrinsically pain.
Most people already established for themselves that training (e.g. in sports, or in higher studies) might be experienced as 'pain', but that the phenomenon is not necessarily experienced as such! 'Joy' and 'enthusiasm' might well be the appropriate labels when studying, while running, while presenting…
'Pain' is not inherent, its conditions are not solely context-dependent, one's mindset plays a critical part in conditioning the experience.
Enlightened beings have ceased suffering, unsatisfactoriness, dukkha… the struggle and rebellion against the world as it is, wishing it to be different.
We all know that Awakened beings don't disappear from the causal web —the Buddha lived for decades after attaining nirvāṇa— and that attaining nirvāṇa lies in relating differently to the world (without lust, aversion, or appropriation of some phenomena as 'mine'), i.e. a different 'mindset'.
Enlightened beings don't suffer, because they don't label phenomena as 'painful'. This doesn't mean they don't relate to phenomena at all… A backache is just part of joyfully cultivating a better world, cleansing old karma, finding out (or showing off) what doesn't work, taking the demerits of others onto oneself (so they don't suffer), etc: no more, no less, than any other intentional generous, compassionate, kind, loving, patient, wise enthusiastic effort.