"All of these have at one time been Why questions, and are now How questions."
Yet "Why" remains unanswered. What benefit does it serve for us to have disease, if disease is in fact only here to kill and make things live a painful life? We can know "How" from any number of positions, but we still can't come up with a scientific REASON for it. Only an explanation of it's mechanics.
Diseases do not exist for our benefit. They are not explained in these terms.