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Phlegm (pronounced /ˈflεm/) is sticky fluid secreted by the mucous membranes of humans and other animals. Its definition is limited to the mucus produced by the respiratory system, excluding that from the nasal passages, and particularly that which is expelled by coughing (sputum). Its composition varies, depending on climate, genetics and state of the immune system, but basically is a water-based gel consisting of glycoproteins, immunoglobulins, lipids, etc.
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Phlegm may be of several different colors.
Phlegm may be a carrier of larvae of intestinal parasites (see hookworm). Bloody sputum can be a symptom of serious disease (such as tuberculosis and lung cancer), but can also be a relatively benign symptom of a minor disease (such as bronchitis). In the latter case, the sputum is normally lightly streaked with blood. Coughing up any significant quantity of blood is always a serious medical condition, and any person who experiences this should seek medical attention.
Humourism is an ancient theory that the human body is filled with four basic substances, called the four humours, which are held in balance when a person is healthy. It is closely related to the ancient theory of the four elements and states that all diseases and disabilities result from an excess or deficit in black bile, bile, phlegm, and blood. Hippocrates, an ancient Greek thinker, is credited for this theory, about 400 BC. It influenced medical thinking for more than 2,000 years, until finally discredited in the 1800s.
Phlegm was thought to be associated with apathetic behavior; this old belief is preserved in the word "phlegmatic".
Sir William Osler’s 1889 Aequanimitas discusses the imperturbability or calmness in a storm required of physicians. "Imperturbability means coolness and presence of mind under all circumstances, calmness amid storm, clearness of judgment in moments of grave peril, immobility, impassiveness, or, to use an old and expressive word, phlegm." This was his farewell speech at the University of Pennsylvania in 1889 before becoming Physician-in-Chief at the recently founded Johns Hopkins Hospital in Baltimore, Maryland. This is from "Celebrating the Contributions of William Osler" in the Alan Mason Chesney Medical Archives of the Johns Hopkins Medical Institutions."