Fever, headache, and neurological problems, while classic, only occur in 20% of people with brain abscess.

The famous triad of fever, headache and focal neurologic findings are highly suggestive of brain abscess. These symptoms are caused by a combination of increased intracranial pressure due to a space-occupying lesion (headache, vomiting, confusion, coma), infection (fever, fatigue etc.) and focal neurologic brain tissue damage (hemiparesis, aphasia etc.).

The most frequent presenting symptoms are headache, drowsiness, confusion, seizures, hemiparesis or speech difficulties together with fever with a rapidly progressive course. Headache is characteristically worse at night and in the morning, as the intracranial pressure naturally increases when in the supine position. This elevation similarly stimulates the medullary vomiting center and area postrema, leading to morning vomiting.

Other symptoms and findings depend largely on the specific location of the abscess in the brain. An abscess in the cerebellum, for instance, may cause additional complaints as a result of brain stem compression and hydrocephalus. Neurological examination may reveal a stiff neck in occasional cases (erroneously suggesting meningitis).

Brain abscess (or cerebral abscess) is an abscess caused by inflammation and collection of infected material, coming from local (ear infection, dental abscess, infection of paranasal sinuses, infection of the mastoid air cells of the temporal bone, epidural abscess) or remote (lung, heart, kidney etc.) infectious sources, within the brain tissue. The infection may also be introduced through a skull fracture following a head trauma or surgical procedures. Brain abscess is usually associated with congenital heart disease in young children. It may occur at any age but is most frequent in the third decade of life.

A cranial epidural abscess involves pus and granulation tissue accumulation in between the dura mater and cranial bone. These typically arise (along with osteomyelitis of a cranial bone) from infections of the ear or paranasal sinuses. They rarely can be caused by distant infection or an infected cerebral venous sinus thrombosis. Staphylococcus aureus is the most common pathogen. Symptoms include pain at the forehead or ear, pus draining from the ear or sinuses, tenderness overlying the infectious site, fever, neck stiffness, and in rare cases focal seizures. Treatment requires a combination of antibiotics and surgical removal of infected bone.

Spinal epidural abscess (SEA) is a collection of pus or inflammatory granulation between the dura mater and the vertebral column. Currently the annual incidence rate of SEAs is estimated to be 2.5-3 per 10,000 hospital admissions. Incidence of SEA is on the rise, due to factors such as an aging population, increase in use of invasive spinal instrumentation, growing number of patients with risk factors such as diabetes and intravenous drug use. SEAs are more common in posterior than anterior areas, and the most common location is the thoracolumbar area, where epidural space is larger and contains more fat tissue.

SEAs are more common in males, and can occur in all ages, although highest prevalence is during the fifth and seventh decades of life.

Abscesses may occur in any kind of solid tissue but most frequently on skin surface (where they may be superficial pustules (boils) or deep skin abscesses), in the lungs, brain, teeth, kidneys, and tonsils. Major complications are spreading of the abscess material to adjacent or remote tissues, and extensive regional tissue death (gangrene).

The main symptoms and signs of a skin abscess are redness, heat, swelling, pain, and loss of function. There may also be high temperature (fever) and chills.

An internal abscess is more difficult to identify, but signs include pain in the affected area, a high temperature, and generally feeling unwell.

Internal abscesses rarely heal themselves, so prompt medical attention is indicated if such an abscess is suspected.

An abscess could potentially be fatal (although this is rare) if it compresses vital structures such as the trachea in the context of a deep neck abscess.

If superficial, abscesses may be fluctuant when palpated. This is a wave-like motion that is caused by movement of the pus inside the abscess.

An amoebic brain abscess is a brain abscess caused by amoebas. It is particularly serious in patients with immunodeficiency. It is very rare; the first case was reported in 1849.

It may be caused by "Entamoeba histolytica".

The clinical presentation of CST can be varied. Both acute, fulminant disease and indolent, subacute presentations have been reported in the literature.

The most common signs of CST are related to anatomical structures affected within the cavernous sinus, notably cranial nerves III-VI, as well as symptoms resulting from impaired venous drainage from the orbit and eye.

Classic presentations are abrupt onset of unilateral periorbital edema, headache, photophobia, and bulging of the eye (proptosis).

Other common signs and symptoms include:

Ptosis, chemosis, cranial nerve palsies (III, IV, V, VI). Sixth nerve palsy is the most common. Sensory deficits of the ophthalmic and maxillary branch of the fifth nerve are common. Periorbital sensory loss and impaired corneal reflex may be noted. Papilledema, retinal hemorrhages, and decreased visual acuity and blindness may occur from venous congestion within the retina. Fever, tachycardia and sepsis may be present. Headache with nuchal rigidity may occur. Pupil may be dilated and sluggishly reactive. Infection can spread to contralateral cavernous sinus within 24–48 hours of initial presentation.

Surgery of the anal fistula to drain an abscess treats the fistula and reduces likelihood of its recurrence and the need for repeated surgery. There is no evidence that fecal incontinence is a consequence of this surgery for abscess drainage.

Perianal abscesses can be seen in patients with for example inflammatory bowel disease (such as Crohn's disease) or diabetes. Often the abscess will start as an internal wound caused by ulceration, hard stool or penetrative objects with insufficient lubrication. This wound typically becomes infected as a result of the normal presence of feces in the rectal area, and then develops into an abscess. This often presents itself as a lump of tissue near the anus which grows larger and more painful with time. Like other abscesses, perianal abscesses may require prompt medical treatment, such as an incision and debridement or lancing.

Cavernous sinus thrombosis (CST) is the formation of a blood clot within the cavernous sinus, a cavity at the base of the brain which drains deoxygenated blood from the brain back to the heart. The cause is usually from a spreading infection in the nose, sinuses, ears, or teeth. "Staphylococcus aureus" and "Streptococcus" are often the associated bacteria. Cavernous sinus thrombosis symptoms include: decrease or loss of vision, chemosis, exophthalmos (bulging eyes), headaches, and paralysis of the cranial nerves which course through the cavernous sinus. This infection is life-threatening and requires immediate treatment, which usually includes antibiotics and sometimes surgical drainage.

Signs and symptoms may include stiff neck (limited neck mobility or torticollis), some form of palpable neck pain (may be in "front of the neck" or around the Adam's Apple), malaise, difficulty swallowing, fever, stridor, drooling, croup-like cough or enlarged cervical lymph nodes. Any combination of these symptoms should arouse suspicion of RPA.

An unremoved infected abscess may lead to sepsis. Also, multiple abscesses may occur. Other complications may include fistula formation and recurrent pancreatitis.

Patients with pancreatic abscesses may experience abdominal pain, chills and fever or the inability to eat. Whereas some patients present an abdominal mass, others do not. Nausea and vomiting may also occur.

Fever, sore throat, odynophagia (painful swallowing), swelling in neck.

Anterior space abscess can cause trismus (spasm of jaw muscle) and hard mass formation along the angle of the mandible, with medial bulging of the tonsil and lateral pharyngeal wall. Posterior space abscess causes swelling in posterior pharyngeal wall. Trismus (spasm of jaw muscle) is minimal. Posterior abscess may involve structures within the carotid sheath, causing rigors, high fever, bacteremia, neurologic deficit, massive haemorrhage caused by carotid artery rupture.

Unlike tonsillitis, which is more common in the children, PTA has a more even age spread, from children to adults. Symptoms start appearing two to eight days before the formation of an abscess. A progressively severe sore throat on one side and pain during swallowing (odynophagia) usually are the earliest symptoms. As the abscess develops, persistent pain in the peritonsillar area, fever, a general sense of feeling unwell, headache and a distortion of vowels informally known as "hot potato voice" may appear. Neck pain associated with tender, swollen lymph nodes, referred ear pain and foul breath are also common. While these signs may be present in tonsillitis itself, a PTA should be specifically considered if there is limited ability to open the mouth (trismus).

Physical signs of a peritonsillar abscess include redness and swelling in the tonsillar area of the affected side and swelling of the jugulodigastric lymph nodes. The uvula may be displaced towards the unaffected side.

Retropharyngeal abscess (RPA) is an abscess located in the tissues in the back of the throat behind the posterior pharyngeal wall (the retropharyngeal space). Because RPAs typically occur in deep tissue, they are difficult to diagnose by physical examination alone. RPA is a relatively uncommon illness, and therefore may not receive early diagnosis in children presenting with stiff neck, malaise, difficulty swallowing, or other symptoms listed below. Early diagnosis is key, while a delay in diagnosis and treatment may lead to death. Parapharyngeal space communicates with retropharyngeal space and an infection of retropharyngeal space can pass down behind the esophagus into the mediastinum. RPAs can also occur in adults of any age.

RPA can lead to airway obstruction or sepsis – both life-threatening emergencies. Fatalities normally occur from patients not receiving treatment immediately and suffocating prior to knowing that anything serious was wrong.

Severe pain in perimastoid region, difficulty of swallowing, sore throat, difficulty in breathing, nuchal rigidity, and fever.

Onset of symptoms is often gradual, but in necrotizing staphylococcal or gram-negative bacillary pneumonias patients can be acutely ill. Cough, fever with shivering, and night sweats are often present. Cough can be productive of foul smelling purulent mucus (≈70%) or less frequently with blood in one third of cases). Affected individuals may also complain of chest pain, shortness of breath, lethargy and other features of chronic illness.

Those with a lung abscess are generally cachectic at presentation. Finger clubbing is present in one third of patients. Dental decay is common especially in alcoholics and children. On examination of the chest there will be features of consolidation such as localized dullness on percussion and bronchial breath sounds.

Localized pain, often nocturnal, alleviated by aspirin. Often mimics the symptoms of Osteoid osteoma, which is typically less than 1 cm in diameter.

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.

This condition is often initially misdiagnosed as hemorrhoids, since this is almost always the cause of any sudden anal discomfort. The presence of the abscess, however, is suspected when the pain quickly worsens over one or two days and usual hemorrhoid treatments are ineffective in bringing relief. Furthermore, any serious abscess will eventually begin to cause signs and symptoms of general infection, including fever and nighttime chills.

A physician can rule out a hemorrhoid with a simple visual inspection, and usually appreciate an abscess by touch.

Pott's puffy tumor, first described by Sir Percivall Pott in 1760, is a rare clinical entity characterized by subperiosteal abscess associated with osteomyelitis. It is characterized by an osteomyelitis of the frontal bone, either direct or through haematogenic spread. This results in a swelling on the forehead, hence the name. The infection can also spread inwards, leading to an intracranial abscess. Pott's puffy tumor can be associated with cortical vein thrombosis, epidural abscess, subdural empyema, and brain abscess. The cause of vein thrombosis is explained by venous drainage of the frontal sinus, which occurs through diploic veins, which communicate with the dural venous plexus; septic thrombi can potentially evolve from foci within the frontal sinus and propagate through this venous system. This type of chronic osteomyelitis of the frontal bone is confused with acute sub-periosteal abscess of the frontal bone, which presents as a discrete collection over the frontal sinus.

Although it can affect all ages, it is mostly found among teenagers and adolescents. It is usually seen as a complication of frontal sinusitis or trauma. Medical imaging can be of use in the diagnosis and evaluation of the underlying cause and extent of the condition. Ultrasound is able to identify frontal bone osteomyelitis, while computed tomography (CT) can evaluate bony erosion, and along with magnetic resonance imaging (MRI), can better appreciate the underlying cause and extent of possible intra-cranial extension/involvement.

A subcutaneous abscess is an abscess located in the subcutaneous tissue (also hypodermis). The abscess is formed due to a hypodermal infection by a bacterium, a fungus or a parasite. Typically, this kind of abscess needs drainage, usually for a minimum of 24 hours, by means of gauze packing or a Penrose drain.

Pain in the perianal area is the most common symptom of an anorectal abscess. The pain may be dull, aching, or throbbing. It is worst when the person sits down and right before a bowel movement. After the individual has a bowel movement, the pain usually lessens. Other signs and symptoms of anorectal abscess include constipation, drainage from the rectum, fever and chills, or a palpable mass near the anus.

The condition invariably becomes extremely painful, and usually worsens over the course of just a few days. The pain may be limited and sporadic at first, but invariably worsens to a constant pain which can become very severe when body position is changed (e.g., when standing up, rolling over, and so forth). Depending upon the exact location of the abscess, there can also be excruciating pain during bowel movements, though this is not always the case. This condition may occur in isolation, but is frequently indicative of another underlying disorder, such as Crohn's disease.

Some common symptoms and signs of mastoiditis include pain, tenderness, and swelling in the mastoid region. There may be ear pain (otalgia), and the ear or mastoid region may be red (erythematous). Fever or headaches may also be present. Infants usually show nonspecific symptoms, including anorexia, diarrhea, or irritability. Drainage from the ear occurs in more serious cases, often manifest as brown discharge on the pillowcase upon waking.