Chronic illness : Introduction Serious chronic illnesses are a major health issue in modern society. Any illness is called "chronic" if it is long-lasting or even lifelong. The opposite of chronic is "acute", referring to diseases that come on quickly and often do not last long (if they last, they are said to become "chronic"). In the USA, more than 90 million people have a chronic illness, and the top 5 chronic illnesses (heart disease, cancer, stroke, COPD, and diabetes) together cause more than two-thirds of all deaths.