Definition of illness noun from the Oxford Advanced American Dictionary

      

    illness

     noun
    noun
    NAmE//ˈɪlnəs//
     
     
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  1. 1[uncountable] the state of being physically or mentally ill mental illness I missed a lot of school through illness last year.
  2. 2[countable] a type or period of illness minor/serious illnesses childhood illnesses He died after a long illness. Thesaurusdiseaseillness disorder infection condition ailment bugThese are all words for a medical problem.disease a medical problem affecting humans, animals, or plants, often caused by infection:He suffers from a rare blood disease.illness a medical problem, or a period of suffering from one:She died after a long illness.disease or illness?Disease is used to talk about more severe physical medical problems, especially those that affect the organs. Illness is used to talk about both more severe and more minor medical problems, and, more commonly, those that affect mental health:heart/kidney/liver illness mental disease. Disease is not used about a period of illness:She died after a long disease.disorder (somewhat formal) an illness that causes a part of the body to stop functioning correctly:a rare disorder of the liver A disorder is generally not infectious. Disorder is used most frequently with words relating to mental problems, for example psychiatric, personality, mental, and eating. When it is used to talk about physical problems, it is most often used with blood, bowel, and kidney, and these are commonly serious, severe, or rare.infection an illness that is caused by bacteria or a virus:a sinus infection an ear infectioncondition a medical problem that you have for a long time because it is not possible to cure it:a heart conditionailment (somewhat formal) an illness that is not very serious:childhood ailmentsbug (informal) an infectious illness that is usually fairly mild:a nasty flu bugPatterns to have/suffer from a(n) disease/illness/disorder/infection/condition/ailment/bug to catch/contract/get/pick up a(n) disease/illness/infection/bug
  3. Thesaurusillnesscondition health problems/issues disabilityThese are all words for the state of someone's health, especially when they are physically or mentally ill.illness the state of being physically or mentally illcondition the state of someone's health or how fit they are, especially when they are ill:She can't travel in her condition. My condition requires me to watch what I eat.health problems/issues any problems that someone has with their physical or mental health:My parents have had a lot of health problems this year. Many of the patients at the clinic are dealing with serious mental health issues. The term health issues is slightly more formal than health problems.disability a physical or mental condition that means you cannot use a part of your body completely or easily, or that you cannot learn easily:His disability does not keep him from living a full and active life.Patterns (a) chronic illness/condition/health problems/health issues/disability to suffer from an illness/health problems/health issues to deal with/have health problems/health issues/a disability
See the Oxford Advanced Learner's Dictionary entry: illness