Pope - can a pope retire?

It seems harsh to keep the present Pontiff in harness when he is so frail and ill. Perhaps he does want to carry on but even so it is a sad situation.
Do you know of a case when a Pope retired from his duties due to illness and if so, who fills in for him. All right, no-one could take his place really but you get what I mean.
And if it hasn’t ever happened, don’t you think they should make contingency plans after so many centuries.



Thanks Revtim. Looks as if some of the precedents were dubious, so this Pope might not find the concept of abdication an honourable one.