>>That's not the same. Invalid is a disabled person - check in other languages. "Bolesnik" is just a person with an illness.
>
>There's always "sufferer": "... The later stages of this disease can bring great pain to the sufferer ..."

...which doesn't imply he's ill. He may just be gently interrogated: "... The later stages of this technique can bring great pain to the sufferer ..."