In a hereditary monarchy, you were ruled by a "rightwise born" king or queen, and no illness, disability, or incompetence could change that. Certainly death could, but not always for the better. Take the case of the Tudor monarchs of 16th century England. The line of succession was drastically altered by illnesses and injuries that medical science could not ease, and so was the behavior of those monarchs.
Henry VII expected his firstborn son Arthur to inherit the crown. At age 15, Arthur was married to Katherine of Aragorn, but the couple soon fell ill of what was called "English sweating sickness." Historians are still not sure what English sweating sickness really was. Arthur died of the illness, leaving his younger brother Henry as the only royal heir. Katherine survived to marry her brother-in-law Henry, who became Henry VIII.
But Henry VIII suffered from other medical problems, as did Katherine, and their offspring, many of whom died before or shortly after birth, leading to a whole nation being jerked around in the quest for a royal heir. Read about ten medical problems that shaped the history of the Tudor dynasty at Mental Floss.
(Image credit: Lucas de Heere)