It's never just heart disease... and rarely just illness
Illnesses in literature can mean more than someone simply being sick. Heart disease is usually a representation of things to do with love, sympathy or compassion either with the world the character is in or the character themselves.
A common message that this is meant to deliver is a lack of sympathy or too much in a person, which causes them to do something E.g visiting and assisting the poor, who pass on said disease, which would be an example of too much. after said event happen, they will start to decline and fade in health, which demonstrates what they were and makes it clear that this characteristic is what killed them, not directly but it is the reason the catch the disease and eventually die.
A common message that this is meant to deliver is a lack of sympathy or too much in a person, which causes them to do something E.g visiting and assisting the poor, who pass on said disease, which would be an example of too much. after said event happen, they will start to decline and fade in health, which demonstrates what they were and makes it clear that this characteristic is what killed them, not directly but it is the reason the catch the disease and eventually die.