Skin abscesses are common and have become more common in recent years. Risk factors include intravenous drug use with rates reported as high as 65% in this population. In 2005 in the United States 3.2 million people went to the emergency department for an abscess. In Australia around 13,000 people were hospitalized in 2008 for the disease.

Risk factors for abscess formation include intravenous drug use. Another possible risk factor is a prior history of disc herniation or other spinal abnormality, though this has not been proven.

Abscesses are caused by bacterial infection, parasites, or foreign substances.

Bacterial infection is the most common cause. Often many different types of bacteria are involved in a single infection. In the United States and many other areas of the world the most common bacteria present is "methicillin-resistant Staphylococcus aureus". Among spinal subdural abscesses, methicillin-sensitive Staphylococcus aureus is the most common organism involved.

Rarely parasites can cause abscesses and this is more common in the developing world. Specific parasites known to do this include dracunculiasis and myiasis.

In some cases, abscesses may be prevented by draining an existing pseudocyst which is likely to become inflamed. However, in most cases the developing of abscesses cannot be prevented.

Pancreatic abscesses usually develop in patients with pancreatic pseudocysts that become infected. They may also form as a result of fibrous wall formation around fluid collections or penetrating peptic ulcers. Other causes include gall stones or alcohol consumption and, in rare cases, drugs, blunt trauma and following extension abscess from nearby structures.

Fungi and parasites may also cause the disease. Fungi and parasites are especially associated with immunocompromised patients. Other causes include: "Nocardia asteroides", "Mycobacterium", Fungi (e.g. "Aspergillus", "Candida", "Cryptococcus", "Mucorales", "Coccidioides", "Histoplasma capsulatum", "Blastomyces dermatitidis", "Bipolaris", "Exophiala dermatitidis", "Curvularia pallescens", "Ochroconis gallopava", "Ramichloridium mackenziei", "Pseudallescheria boydii"), Protozoa (e.g. "Toxoplasma gondii", "Entamoeba histolytica", "Trypanosoma cruzi", "Schistosoma", "Paragonimus"), and Helminths (e.g. "Taenia solium"). Organisms that are most frequently associated with brain abscess in patients with AIDS are poliovirus, "Toxoplasma gondii", and "Cryptococcus neoformans", though in infection with the latter organism, symptoms of meningitis generally predominate.

These organisms are associated with certain predisposing conditions:

- Sinus and dental infections—Aerobic and anaerobic streptococci, anaerobic gram-negative bacilli (e.g. "Prevotella", "Porphyromonas", "Bacteroides"), "Fusobacterium", "S. aureus", and Enterobacteriaceae

- Penetrating trauma—"S. aureus", aerobic streptococci, Enterobacteriaceae, and "Clostridium" spp.

- Pulmonary infections—Aerobic and anaerobic streptococci, anaerobic gram-negative bacilli (e.g. "Prevotella", "Porphyromonas", "Bacteroides"), "Fusobacterium", "Actinomyces", and "Nocardia"

- Congenital heart disease—Aerobic and microaerophilic streptococci, and "S. aureus"

- HIV infection—"T. gondii", "Mycobacterium", "Nocardia", "Cryptococcus", and "Listeria monocytogenes"

- Transplantation—"Aspergillus", "Candida", "Cryptococcus", "Mucorales", "Nocardia", and "T. gondii"

- Neutropenia—Aerobic gram-negative bacilli, "Aspergillus", "Candida", and "Mucorales"

Death occurs in about 10% of cases and people do well about 70% of the time. This is a large improvement from the 1960s due to improved ability to image the head, better neurosurgery and better antibiotics.

When properly diagnosed, the mortality of Lemierre's syndrome is about 4.6%. Since this disease is not well known and often remains undiagnosed, mortality might be much higher.

Abscesses are caused by a high-density infection of (usually) common bacteria which collect in one place or another for any variety of reasons. Anal abscesses, without treatment, are likely to spread and affect other parts of the body, particularly the groin and rectal lumen. All abscesses can progress to serious generalized infections requiring lengthy hospitalizations if not treated.

Historically, many rectal abscesses are caused by bacteria common in the digestive system, such as E. coli. While this still continues often to be the case, there has recently been an uptick in the causative organism being staphylococcus, as well as the difficult to treat community-acquired methicillin-resistant S. aureus. Because of the increasing appearance of more exotic bacteria in anal abscesses, microbiological examination will always be performed on the surgical exudate to determine the proper course of any antibiotic treatment.

The bacteria causing the thrombophlebitis are anaerobic bacteria that are typically normal components of the microorganisms that inhabit the mouth and throat. Species of "Fusobacterium", specifically "Fusobacterium necrophorum", are most commonly the causative bacteria, but various bacteria have been implicated. One 1989 study found that 81% of Lemierres's syndrome had been infected with "Fusobacterium necrophorum", while 11% were caused by other Fusobacterium species. MRSA might also be an issue in Lemierre infections. Rarely Lemierre's syndrome is caused by other (usually Gram-negative) bacteria, which include "Bacteroides fragilis" and "Bacteroides melaninogenicus", "Peptostreptococcus spp.", "Streptococcus microaerophile", "Staphylococcus aureus", "Streptococcus pyogenes", and "Eikenella corrodens".

Both sex are equally affected

Any age group can develop a parapheryngeal abscess but it is most commonly seen in children and adolescents. Adults who are immunocompromised are also at high risk.

If left untreated, an anal fistula will almost certainly form, connecting the rectum to the skin. This requires more intensive surgery. Furthermore, any untreated abscess may (and most likely will) continue to expand, eventually becoming a serious systemic infection.

Subphrenic abscess is a disease characterized by an accumulation of infected fluid between the diaphragm, liver, and spleen. This abscess develops after surgical operations like splenectomy.

Presents with cough, increased respiratory rate with shallow respiration, diminished or absent breath sounds, hiccups, dullness in percussion, tenderness over the 8th–11th ribs, fever, chills, anorexia and shoulder tip pain on the affected side. Lack of treatment or misdiagnosis could quickly lead to sepsis, septic shock, and death. It is also associated with peritonitis.

Infection can occur from:

- Pharynx: acute and chronic infection of tonsil and adenoids

- Teeth: dental infection occurs from lower last molar tooth

- Ear: bezold abscess and petrositis

- Other space: infection of parotid retropharyngeal space

- External trauma: penetrating injuries of neck, injection of local anaesthetic

There are three major forms of liver abscess, classified by cause:

- Pyogenic liver abscess, which is most often polymicrobial, accounts for 80% of hepatic abscess cases in the United States.

- Amoebic liver abscess due to "Entamoeba histolytica" accounts for 10% of cases.

- Fungal abscess, most often due to "Candida" species, accounts for less than 10% of cases.

Major bacterial causes of liver abscess include the following:

- "Streptococcus" species (including "Enterococcus")

- "Escherichia" species

- "Staphylococcus" species

- "Klebsiella" species (Higher rates in the Far East)

- Anaerobes (including "Bacteroides" species)

- "Pseudomonas" species

- "Proteus" species

However, as noted above, many cases are polymicrobial.

In the post-antibiotic era pattern of frequency is changing. In older studies anaerobes were found in up to 90% cases but they are much less frequent now.

Lung abscess is a type of liquefactive necrosis of the lung tissue and formation of cavities (more than 2 cm) containing necrotic debris or fluid caused by microbial infection.

This pus-filled cavity is often caused by aspiration, which may occur during anesthesia, sedation, or unconsciousness from injury. Alcoholism is the most common condition predisposing to lung abscesses.

Lung abscess is considered primary (60%) when it results from existing lung parenchymal process and is termed secondary when it complicates another process e.g. vascular emboli or follows rupture of extrapulmonary abscess into lung.

PTA usually arises as a complication of an untreated or partially treated episode of acute tonsillitis. The infection, in these cases, spreads to the peritonsillar area (peritonsillitis). This region comprises loose connective tissue and is hence susceptible to formation of an abscess. PTA can also occur "". Both aerobic and anaerobic bacteria can be causative. Commonly involved aerobic pathogens include "Streptococcus, Staphylococcus" and "Haemophilus". The most common anaerobic species include "Fusobacterium necrophorum", " Peptostreptococcus", "Prevotella species", and "Bacteroides".

The epidemiology of TOA is closely related to that of pelvic inflammatory disease which is estimated to one million people yearly.

Peritonsillar abscess (PTA), also known as a quinsy, is pus due to an infection behind the tonsil. Symptoms include fever, throat pain, trouble opening the mouth, and a change to the voice. Pain is usually worse on one side. Complications may include blockage of the airway or aspiration pneumonitis.

They are typically due to infection by a number of types of bacteria. Often it follows streptococcal pharyngitis. They do not typically occur in those who have had a tonsillectomy. Diagnosis is usually based on the symptoms. Medical imaging may be done to rule out complications.

Treatment is by removing the pus, antibiotics, sufficient fluids, and pain medication. Steroids may also be useful. Admission to hospital is generally not needed. In the United States about 3 per 10,000 people per year are affected. Young adults are most commonly affected.

Complications of TOA are related to the possible removal of one or both ovaries and fallopian tubes. Without these reproductive structures, fertility can be affected. Surgical complications can develop and include:

- Allergic shock due to anesthetics

- A paradoxical reaction to a drug

- Infection

With prompt treatment, it is possible to cure mastoiditis. Seeking medical care early is important. However, it is difficult for antibiotics to penetrate to the interior of the mastoid process and so it may not be easy to cure the infection; it also may recur. Mastoiditis has many possible complications, all connected to the infection spreading to surrounding structures. Hearing loss is likely, or inflammation of the labyrinth of the inner ear (labyrinthitis) may occur, producing vertigo and an ear ringing may develop along with the hearing loss, making it more difficult to communicate. The infection may also spread to the facial nerve (cranial nerve VII), causing facial-nerve palsy, producing weakness or paralysis of some muscles of facial expression, on the same side of the face. Other complications include Bezold's abscess, an abscess (a collection of pus surrounded by inflamed tissue) behind the sternocleidomastoid muscle in the neck, or a subperiosteal abscess, between the periosteum and mastoid bone (resulting in the typical appearance of a protruding ear). Serious complications result if the infection spreads to the brain. These include meningitis (inflammation of the protective membranes surrounding the brain), epidural abscess (abscess between the skull and outer membrane of the brain), dural venous thrombophlebitis (inflammation of the venous structures of the brain), or brain abscess.

A amoebic liver abscess is a type of liver abscess caused by amebiasis. It is the involvement of liver tissue by trophozoites of the organism "Entamoeba histolytica" and of is abscess due to necrosis.

Treatment must also include a lumenal amoebicide to prevent reinvasion of tissues by amoebae still in the intestines (see Amoebiasis). After completion of treatment with tissue amebicides, administer luminal amebicides for eradication of the asymptomatic colonization state. Failure to use luminal agents can lead to relapse of infection in approximately 10% of patients.

In the United States and other developed countries, the incidence of mastoiditis is quite low, around 0.004%, although it is higher in developing countries. The condition most commonly affects children aged from two to thirteen months, when ear infections most commonly occur. Males and females are equally affected.