Pus is a viscous exudate

Exudate

An exudate is any fluid that filters from the circulatory system into lesions or areas of inflammation. It can apply to plants as well as animals. Its composition varies but generally includes water and the dissolved solutes of the main circulatory fluid such as sap or blood...

, typically whitish-yellow, yellow, or yellow-brown, formed at the site of inflammatory

Inflammation

Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

 during infection

Infection

An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

. An accumulation of pus in an enclosed tissue space is known as an abscess

Abscess

An abscess is a collection of pus that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process or other foreign materials...

, whereas a visible collection of pus within or beneath the epidermis is known as a pustule or pimple

Pimple

A pimple, zit or spot is a kind of acne, and one of the many results of excess oil getting trapped in the pores. Some of the varieties are pustules or papules....

.

Pus consists of a thin, protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

-rich fluid, known as liquor puris, and dead leukocyte of the body's immune response (mostly neutrophils). During infection, macrophage

Macrophage

Macrophages are cells produced by the differentiation of monocytes in tissues. Human macrophages are about in diameter. Monocytes and macrophages are phagocytes. Macrophages function in both non-specific defense as well as help initiate specific defense mechanisms of vertebrate animals...

s release cytokine

Cytokine

Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

s which trigger neutrophils to seek the site of infection by chemotaxis

Chemotaxis

Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...

. There, the neutrophils engulf and destroy the bacteria and the bacteria resist the immune response by releasing toxins called leukocidin

Leukocidin

A leukocidin is a type of cytotoxin created by some types of bacteria. It is a type of pore forming toxin. Leukocidins get their names by killing leukocytes.One type is Panton-Valentine leukocidin....

s.. As the neutrophils die off from toxins and old age, they are destroyed by macrophages, forming the viscous pus.

Bacteria that cause pus are called suppurative, pyogenic, or purulent. If the agent also creates mucus

Mucus

In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

, it is called mucopurulent. Purulent infections can be treated with an antiseptic

Antiseptic

Antiseptics are antimicrobial substances that are applied to living tissue/skin to reduce the possibility of infection, sepsis, or putrefaction...

.

Despite normally being of a whitish-yellow hue, changes in the color of pus can be observed under certain circumstances. Pus is sometimes green because of the presence of myeloperoxidase

Myeloperoxidase

Myeloperoxidase is a peroxidase enzyme that in humans is encoded by the MPO gene. Myeloperoxidase is most abundantly expressed in neutrophil granulocytes . It is a lysosomal protein stored in azurophilic granules of the neutrophil...

, an intensely green antibacterial protein produced by some types of white blood cells. Blue-green pus is found in certain infections of Pseudomonas aeruginosa

Pseudomonas aeruginosa

Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...

as a result of the pyocyanin bacterial pigment it produces; amoebic

Amoeba

Amoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...

 abscesses of the liver

Liver

The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 produce brownish pus. Pus can also have a foul odor.

In almost all cases when there is a collection of pus in the body, the clinician will try to create an opening for it to evacuate - this principle has been distilled into the famous Latin aphorism "Ubi pus, ibi evacua

Ubi pus, ibi evacua

Ubi pus, ibi evacua is a Latin aphorism or adage, often cited in medicine, meaning "where [there is] pus, there evacuate [it]". It refers to what clinicians should do when there is a collection of pus in the body; that is, to create an opening for it to evacuate...

!"

Some common disease processes caused by pyogenic infections are impetigo

Impetigo

Impetigo is a highly contagious bacterial skin infection most common among pre-school children. People who play close contact sports such as rugby, American football and wrestling are also susceptible, regardless of age. Impetigo is not as common in adults. The name derives from the Latin impetere...

, osteomyelitis

Osteomyelitis

Osteomyelitis simply means an infection of the bone or bone marrow...

, septic arthritis

Septic arthritis

Septic arthritis is the purulent invasion of a joint by an infectious agent which produces arthritis. People with artificial joints are more at risk than the general population but have slightly different symptoms, are infected with different organisms and require different treatment. Septic...

, and necrotizing fasciitis

Necrotizing fasciitis

Necrotizing fasciitis , commonly known as flesh-eating disease or Flesh-eating bacteria syndrome, is a rare infection of the deeper layers of skin and subcutaneous tissues, easily spreading across the fascial plane within the subcutaneous tissue.Necrotizing fasciitis is a quickly progressing and...

.

Pyogenic bacteria

A great many species of bacteria may be pyogenic. The most commonly found include:

• Staphylococcus aureus
  Staphylococcus aureus
  Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...
  
  
• Staphylococcus epidermidis
  Staphylococcus epidermidis
  Staphylococcus epidermidis is one of thirty-three known species belonging to the genus Staphylococcus. It is part of human skin flora, and consequently part of human flora. It can also be found in the mucous membranes and in animals. Due to contamination, it is probably the most common species...
  
  
• Streptococcus pyogenes
  Streptococcus pyogenes
  Streptococcus pyogenes is a spherical, Gram-positive bacterium that is the cause of group A streptococcal infections. S. pyogenes displays streptococcal group A antigen on its cell wall. S...
  
  
• Escherichia coli
  Escherichia coli
  Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...
  
  (Bacillus coli communis)
• Streptococcus pneumoniae
  Streptococcus pneumoniae
  Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...
  
  (Fraenkel's
  Albert Fraenkel
  Albert Fraenkel was a German physician who helped establish Streptococcus pneumoniae as a cause of bacterial pneumonia and championed intravenous ouabain for use in heart failure...
  
   pneumococcus)
• Klebsiella pneumoniae
  Klebsiella pneumoniae
  Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic, rod shaped bacterium found in the normal flora of the mouth, skin, and intestines....
  
  (Friedländer's
  Carl Friedländer
  Carl Friedländer was a German pathologist and microbiologist who helped discover the bacterial cause of pneumonia in 1882...
  
   bacillus)
• Salmonella typhi (Bacillus typhosus)
• Pseudomonas aeruginosa
  Pseudomonas aeruginosa
  Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...
  
  

• Neisseria gonorrhoeae
  Neisseria gonorrhoeae
  Neisseria gonorrhoeae, also known as gonococci , or gonococcus , is a species of Gram-negative coffee bean-shaped diplococci bacteria responsible for the sexually transmitted infection gonorrhea.N...
  
  
• Actinomyces
  Actinomyces
  Actinomyces from Greek "actino" that means mucus and fungus, is a genus of the actinobacteria class of bacteria. They are all Gram-positive and are characterized by contiguous spread, suppurative and granulomatous inflammation, and formation of multiple abscesses and sinus tracts that may...
  
  
• Burkholderia mallei
  Burkholderia mallei
  Burkholderia mallei is a gram-negative bipolar aerobic bacterium, a Burkholderia-genus human and animal pathogen causing Glanders; the Latin name of this disease gave name to the causative agent species...
  
  (Glanders bacillus)
• Mycobacterium tuberculosis
  Mycobacterium tuberculosis
  Mycobacterium tuberculosis is a pathogenic bacterial species in the genus Mycobacterium and the causative agent of most cases of tuberculosis . First discovered in 1882 by Robert Koch, M...
  
  (tubercle bacillus)