What does illness mean?
Illness is a general word for a condition in which a person is not healthy, whether because of a physical disease, a mental disorder, or another impairment. It can refer to a particular diagnosis, as in “a serious illness,” or to the broader state of being unwell, as in “a period of illness.” The word is widely used in medicine, public health, biography, and everyday discussion, though it often sounds more formal than “sickness.” Its usefulness lies in its broad scope: it can describe a specific disease without requiring the diagnosis, while also covering conditions whose exact causes or names are not established. Although often associated with physical health, illness can also refer to mental illness, making it a useful and inclusive term.
nounA condition in which a person is not healthy, especially a disease or disorder affecting the body or mind. The word can also refer more generally to the state of being ill.
- A disease, disorder, or other condition of poor physical or mental health.
- The state or condition of being ill, especially in contrast to health.
"Her recovery from a prolonged illness inspired the entire community."
"The doctor explained that the illness was mild and should resolve without medication."
"Access to clean water has transformed public health in communities once devastated by childhood illness."
“Illnesses” is the standard plural for countable uses, such as “different illnesses,” while “illness” can also be used uncountably to refer to the general condition of being ill.
"Many common illnesses spread easily in crowded indoor spaces."
Illness can describe a whole medical condition, a particular diagnosis, or simply the state of not being well.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Where does illness come from?
Illness is a Middle English formation built from “ill” and the suffix “-ness,” which creates a noun meaning the state or condition of being ill. The adjective “ill” is an old English word descended from Germanic forms meaning bad, harmful, or unwell. Its Germanic ancestry also connects it with related words and compounds such as “evil,” while the modern derivatives “ill-advised,” “ill-fated,” and “ill-bred” preserve the older sense of badness or misfortune.
Related word forms
How illness is actually used
Illness is neutral to formal in register and is especially common in medical, public-health, and biographical contexts. It may be used as a count noun with an article, as in an illness, or as an uncountable noun referring to the general condition of being ill; “sickness” is often more familiar in everyday speech.
Easily confused with illness
Illness is a broad term for a condition in which someone is not healthy, whereas disease more often refers to a specific disorder with an identifiable medical nature.
Illness usually names a disease or health condition, while sickness can mean either a particular illness or the general experience of feeling unwell.