Answer: All sicknesses that lead to physical and/or mental suffering involve negative karma, as expressed through biological and/or psychological processes naturally. Karma is part of nature too; it is not supernatural.
Not all of the elderly will get seriously sick before departure, as some depart peacefully, relatively or totally free from sickness, with good spiritual practice. This also means they did not have the karma to suffer from serious sickness in their last days. Good spiritual practice can also dissolve negative karma to lighten suffering from sicknesses.
Even the eventual malfunctioning of the body is karmic in nature, as it is the karma of all unenlightened beings to undergo ageing, sickness and death, if they do not practise the Dharma to transcend these in time. Thus does Buddhism call this body that we have a ‘(negative) karmic retribution body’ (业报身), in contrast to the ‘reward body’ (报身) we attain in Pure Land, with Āmítuófó’s blessings.
Even having heart problems due to a poor diet filled with animal products and by-products is karmic in nature. (Notice that it is now occurring to the much younger more these days, with their first heart attack being their last.) Negative karma in this case could have been created by greed, hatred and delusion. There might be greed for ill foods, hatred of healthy vegan foods (i.e. WholeFood Plant-Based Diet: WFPBD), and delusion about proper nutrition – all at the same time. (Cancer is also avoidable with a good diet and good spiritual practice.)
Please Be Mindful Of Your Speech, Namo Amituofo!