Explore comprehensive guides on hundreds of common and rare diseases and conditions from the experts at Mayo Clinic. A to Z list of common illnesses and conditions including their symptoms, causes and treatments. Illness is a broad term that defines the poor state of mind, body, and, to a certain extent, spirit. It is the general feeling of being sick or unwell (outside the person’s belief of good health). (Definition of illness from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press) Illness is a state of poor health or sickness, as in I’ve had to miss a lot of work due to illness. The word illness is also commonly used to refer to a disease or instance of sickness. An illness is any state in which a person feels unwell, functions poorly, or experiences symptoms that disrupt normal life. That sounds simple, but the word carries more weight than most people realize. The meaning of ILLNESS is sickness. In humans, disease is often used more broadly to refer to any condition that causes or can cause pain, dysfunction, distress, social problems, or death to the person affected, or similar problems for those in contact with the person. Find out what's going around near you right now — flu activity, COVID-19 levels, RSV, measles, and local health context for all 50 states and 499 U.S. cities. If you have an illness, there is something wrong with your health. An illness can last for a long time or a short time, and its effects can be serious or not serious.