My Mum seemed as fit as a fiddle; she was out walking most days or going off to various clubs she belonged to. No sign of any illness. Then a couple of times she said she felt a bit tired; about a week later she said she was feeling worse and could hardly move, so I called an ambulance. They took her into hospital where she was told she had terminal cancer. She died about four days later. From being seemingly fit with no problems at all to dying must have been no more than a month at most. Oh yes, it can happen suddenly.