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Phlegm (pronounced /flɛm/) is sticky fluid secreted by the mucus 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 in glycoproteins, immunoglobulins, lipids, etc.

In Hippocratic medicine, for hundreds of years until about the 19th century, phlegm was counted as one of the four bodily humours, possessing the properties of coldness and wetness, and was responsible for apathetic and sluggish behaviour. This old belief is preserved in the word phlegmatic.

Phlegm may be of several different colors.

• "Healthy" phlegm is normally clear or white however the initial state of the common flu when the phlegm is still clear can also be the most infectious period.
• Yellow phlegm is normally a sign of an infection and means the immune system is responding.^{[citation needed]}
• Greenish or brownish phlegm is nearly always a sign of infection. Greenish or rusty phlegm or phlegm with rusty spots can also be a sign of pneumonia and/or internal micro-bleedings.^{[citation needed]}
• Coughing up brown phlegm is also a common symptom of smoking. This is due to resin sticking to the viscous texture of the phlegm and being ejected by the body.^{[citation needed]}
• Another type of phlegm often associated with smoking is brownish gray in color. This variant is encased in clear saliva. When spread out, the brown-gray "core" is shown to be grainy in composition, as opposed to holding together. This is simply dust and other foreign matter and may be caused by damage to the cilia, as in COPD patients.^{[citation needed]}

Plegm sometimes gets caught in the throatal region of the body. When one coughs extra hard they can cough up the phlegm.

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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. Another case would be if you had a nose bleed and some blood went down your throat. Some blood could stick to some phlegm and cause bloody phlegm.

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 states that all diseases and disabilities result from an excess or deficit in black bile, bile, phlegm, and blood.