Anaerobes can be isolated from most types of upper respiratory tract and head and neck and infection and are especially common in chronic ones. These include tonsillar, peritonsillar and retropharyngeal abscesses, chronic otitis media, sinusitis and mastoiditis, eye ocular) infections, all deep neck space infections, parotitis, sialadenitis, thyroiditis, odontogenic infections, and postsurgical and nonsurgical head and neck wounds and abscesses., The predominant organisms are of oropharyngeal flora origin and include AGNB, "Fusobacterium" and Peptostreptococcus spp.

Anaerobes involve almost all dental infections. These include dental abscesses, endodontal pulpitis and periodontal (gingivitis and periodontitis) infections, and perimandibular space infection. Pulpitis can lead to abscess formation and eventually spread to the mandible and other neck spaces. In addition to strict anaerobic bacteria, microaerophilic streptococci and "Streptococcus salivarius" can also be present.

"Fusobacterium" spp. and anaerobic spirochetes are often the cause of acute necrotizing ulcerative gingivitis (or Vincent's angina) which is a distinct form of ulcerative gingivitis.

Deep neck infections that develop as a consequence of oral, dental and pharyngeal infections are generally polymicrobial in nature. These include extension of retropharyngeal cellulitis or abscess, mediastinitis following esophagus perforation, and dental or periodontal abscess.

The infections that frequently involve anaerobic bacteria include superficial infections, including infected paronychia, infected human or animal bites, cutaneous ulcers, cellulitis, pyoderma, and hidradenitis suppurativa. Secondary infected sites include secondary infected diaper rash, gastrostomy or tracheostomy site wounds, scabies or kerion infections, eczema, psoriasis, poison ivy, atopic dermatitis, eczema herpeticum, infected subcutaneous sebaceous or inclusion cysts, and postsurgical wound infection.

Skin involvement in subcutaneous tissue infections includes: cutaneous and subcutaneous abscesses, breast abscess, decubitus ulcers, infected pilonidal cyst or sinus, Meleney's ulcer infected diabetic (vascular or trophic) ulcers, bite wound, anaerobic cellulitis and gas gangrene, bacterial synergistic gangrene, and burn wound infection. Deeper anaerobic soft-tissue infections are necrotizing fasciitis, necrotizing synergistic cellulitis, gas gangrene and crepitus cellulitis. These can involve the fascia as well as the muscle surrounded by the fascia, and may also induce myositis and myonecrosis.

The isolates found in soft-tissue infections can vary depending on the type of infection. The infection's location and the circumstances causing the infection can also influence the nature of the microorganisms recovered. Bacteria that are members of the 'normal flora' of the region of the infection are often also isolated from lesions involving anaerobic bacteria.

Specimens obtained from wounds and subcutaneous tissue infections and abscesses in the rectal area (perirectal abscess, decubitus ulcer) or that are of gut flora origin(i.e. diabetic foot infection) often to yield colonic flora organisms. These are generally "B. fragilis" group, "Clostridium" spp., Enterobacteriaceae and "Enterococcus" spp. On the other hand, infections in and around the oropharynx, or infections that originate from that location, frequently contain oral flora organisms (i.e. paronychia, bites, breast abscess). These bacteria include pigmented "Prevotella" and "Porphyromonas, Fusobacterium" and Peptostreptococcus spp. Skin flora organisms such as "S. aureus" and "Streptococcus" spp., or nosocomially acquired microorganisms can be recovered at all body locations. Human bite infections often contain "Eikenella" spp. and animal bites harbor "Pasteurella multocida" in addition to oral flora,

Anaerobes infections are often polymicrobial in nature, and sometimes (i.e. decubitus ulcers, diabetic foot ulcer) they are complicated by bacteremia and or osteomyelitis . Infections which are in the deep tissues ( necrotizing cellulitis, fasciitis and myositis) often include "Clostridium" spp., "S. pyogenes" or polymicrobic combinations of both aerobic and anaerobic bacteria. Gas in the tissues and putrid-like pus with a gray thin quality are often found in these infections, and they are frequently associated with a bacteremia and high mortality rate.

Treatment of deep-seated soft-tissue infections includes: vigorous surgical management that includes surgical debridement and drainage. Even though there are no controlle studies that support this approach improvement of the involved tissues oxygenation by enhancement of blood supply and administration of hyperbaric oxygen, especially in clostridial infection, may be helpful.

A carbuncle is a cluster of several boils, which is typically filled with purulent exudate (dead neutrophils, phagocitized bacteria, & other cellular components). Fluid may drain freely from the carbuncle, or intervention involving an incision and drainage procedure may be needed. Carbuncles may develop anywhere, but they are most common on the back and the nape of the neck.

A carbuncle is palpable and can range in size to be as small as a pea or as large as a golf ball. As the impending infection develops, itching may occur. There may be localized erythema, skin irritation, and the area may be painful when touched. Sometimes more severe symptoms may occur, such as fatigue, fever, chills, and malaise.

The initial cause of a carbuncle can often not be determined. Triggers that make carbuncle infections more likely include recent incidence of folliculitis; friction from clothing or shaving; having hair pulled out, such as sites where clothing or furniture grab at hairs; generally poor hygiene; poor nutrition; or weakened immunity. Poor health may be a predisposing factor – for example, persons with diabetes and immune system diseases are more likely to develop infections (especially bacterial infections of the leg or foot).

This most common form of impetigo, also called nonbullous impetigo, most often begins as a red sore near the nose or mouth which soon breaks, leaking pus or fluid, and forms a honey-colored scab, followed by a red mark which heals without leaving a scar. Sores are not painful, but they may be itchy. Lymph nodes in the affected area may be swollen, but fever is rare. Touching or scratching the sores may easily spread the infection to other parts of the body.

Skin ulcers with redness and scarring also may result from scratching or abrading the skin.

A skin and skin structure infection (SSSI), also referred to as skin and soft tissue infection (SSTI) or acute bacterial skin and skin structure infection (ABSSSI), is an infection of skin and associated soft tissues (such as loose connective tissue and mucous membranes). The pathogen involved is usually a bacterial species. Such infections often requires treatment by antibiotics.

Until 2008, two types were recognized, complicated skin and skin structure infection (cSSSI) and uncomplicated skin and skin structure infection (uSSSI). "Uncomplicated" SSSIs included simple abscesses, impetiginous lesions, furuncles, and cellulitis. "Complicated" SSSIs included infections either involving deeper soft tissue or requiring significant surgical intervention, such as infected ulcers, burns, and major abscesses or a significant underlying disease state that complicates the response to treatment. Superficial infections or abscesses in an anatomical site, such as the rectal area, where the risk of anaerobic or gram-negative pathogen involvement is higher, should be considered complicated infections. The two categories had different regulatory approval requirements. The uncomplicated category (uSSSI) is normally only caused by "Staphylococcus aureus" and "Streptococcus pyogenes", whereas the complicated category (cSSSI) might also be caused by a number of other pathogens. In cSSSI, the pathogen is known in only about 40% of cases.

Because cSSSIs are usually serious infections, physicians do not have the time for a culture to identify the pathogen, so most cases are treated empirically, by choosing an antibiotic agent based on symptoms and seeing if it works. For less severe infections, microbiologic evaluation via tissue culture has been demonstrated to have high utility in guiding management decisions. To achieve efficacy, physicians use broad-spectrum antibiotics. This practice contributes in part to the growing incidence of antibiotic resistance, a trend exacerbated by the widespread use of antibiotics in medicine in general. The increased prevalence of antibiotic resistance is most evident in methicillin-resistant "Staphylococcus aureus" (MRSA). This species is commonly involved in cSSSIs, worsening their prognosis, and limiting the treatments available to physicians. Drug development in infectious disease seeks to produce new agents that can treat MRSA.

Since 2008, the U.S. Food and Drug Administration has changed the terminology to "acute bacterial skin and skin structure infections" (ABSSSI). The Infectious Diseases Society of America (IDSA) has retained the term "skin and soft tissue infection".

Bullous impetigo, mainly seen in children younger than 2 years, involves painless, fluid-filled blisters, mostly on the arms, legs, and trunk, surrounded by red and itchy (but not sore) skin. The blisters may be large or small. After they break, they form yellow scabs.

A group A streptococcal infection is an infection with group A streptococcus (GAS). "Streptococcus pyogenes" comprises the vast majority of the Lancefield group A streptococci, and is often used as a synonym for GAS. However, "S. dysgalactiae" can also be group A. "S. pyogenes" is a beta-hemolytic species of Gram positive bacteria that is responsible for a wide range of both invasive and noninvasive infections.

Infection of GAS may spread through direct contact with mucus or sores on the skin. GAS infections can cause >500,000 deaths per year. Despite the emergence of antibiotics as a treatment for group A β-hemolytic streptococcus, infection of GAS is an increasing problem, particularly on the continent of Africa.

There are many other types of streptococci (species of "Streptococcus"), including group B streptococcus ("Streptococcus agalactiae") and "Streptococcus pneumoniae", which cause other types of infections and should not be confused with group A strep. Several virulence factors contribute to the pathogenesis of GAS, such as M protein, hemolysins, and extracellular enzymes.

An opportunistic infection is an infection caused by pathogens (bacteria, viruses, fungi, or protozoa) that take advantage of an opportunity not normally available, such as a host with a weakened immune system, an altered microbiota (such as a disrupted gut flora), or breached integumentary barriers. Many of these pathogens do not cause disease in a healthy host that has a normal immune system. However, a compromised immune system, a penetrating injury, or a lack of competition from normal commensals presents an opportunity for the pathogen to infect.

Group A β-hemolytic streptococcus can cause infections of the throat and skin. These may vary from very mild conditions to severe, life-threatening diseases. Although it is not completely clear what causes different people to develop different diseases as a result of infection with the same pathogenic bacteria, it is suspected that host phenotypic and epigenetic factors are the source of such variation. Indeed, the many virulence factors of GAS can influence the epigenetics of the host. Furthermore, persons with suppressed or compromised immune systems may be more susceptible to certain diseases caused by GAS than other persons with intact immune systems.

Humans may also carry the GAS either on the skin or in the throat and show no symptoms. These carriers are less contagious than symptomatic carriers of the bacteria.

The non-invasive infections caused by GAS tend to be less severe and more common. They occur when the bacteria colonizes the throat area, where it recognizes epithelial cells. The two most prominent infections of GAS are both non-invasive: strep throat (pharyngitis) where it causes 15- 30% of the childhood cases and 10% of adult cases, and impetigo. These may be effectively treated with antibiotics. Scarlet fever is also a non-invasive infection caused by GAS, although much less common.

The invasive infections caused by Group A β-hemolytic streptococcus tend to be more severe and less common. These occurs when the bacterium is able to infect areas where bacteria are not usually found, such as blood and organs. The diseases that may be caused as a result of this include streptococcal toxic shock syndrome (STSS), necrotizing fasciitis (NF), pneumonia, and bacteremia.

In addition, infection of GAS may lead to further complications and health conditions, namely acute rheumatic fever and poststreptococcal glomerulonephritis.

Most Common:

- impetigo, cellulitis, and erysipelas – infections of the skin which can be complicated by necrotizing fasciitis – skin, fascia and muscle

- strep throat AKA strep pharyngitis – pharynx

Less Common:

- Bacteremia can be associated with these infections, but is not typical.

- septic arthritis – joints

- osteomyelitis – bones

- vaginitis – vagina (more common in pre-pubescent girls)

- meningitis* – meninges

- sinusitis* – sinuses

- pneumonia* – pulmonary alveolus

The typical signs and symptoms of cellulitis is an area which is red, hot, and painful. The photos shown here of are of mild to moderate cases, and are not representative of earlier stages of the condition.

Bacteremia (also bacteraemia) is the presence of bacteria in the blood. Blood is normally a sterile environment, so the detection of bacteria in the blood (most commonly accomplished by blood cultures) is always abnormal. It is distinct from sepsis, which is the host response to the bacteria.

Bacteria can enter the bloodstream as a severe complication of infections (like pneumonia or meningitis), during surgery (especially when involving mucous membranes such as the gastrointestinal tract), or due to catheters and other foreign bodies entering the arteries or veins (including during intravenous drug abuse). Transient bacteremia can result after dental procedures or brushing of teeth.

Bacteremia can have several important health consequences. The immune response to the bacteria can cause sepsis and septic shock, which has a high mortality rate. Bacteria can also spread via the blood to other parts of the body (which is called hematogenous spread), causing infections away from the original site of infection, such as endocarditis or osteomyelitis. Treatment for bacteremia is with antibiotics, and prevention with antibiotic prophylaxis can be given in high risk situations.

Bacteremia is the presence of bacteria in the bloodstream that are alive and capable of reproducing. It is a type of bloodstream infection. Bacteremia is defined as either a primary or secondary process. In primary bacteremia, bacteria have been directly introduced into the bloodstream. Injection drug use may lead to primary bacteremia. In the hospital setting, use of blood vessel catheters contaminated with bacteria may also lead to primary bacteremia. Secondary bacteremia occurs when bacteria have entered the body at another site, such as the cuts in the skin, or the mucous membranes of the lungs (respiratory tract), mouth or intestines (gastrointestinal tract), bladder (urinary tract), or genitals. Bacteria that have infected the body at these sites may then spread into the lymphatic system and gain access to the bloodstream, where further spread can occur.

Bacteremia may also be defined by the timing of bacteria presence in the bloodstream: transient, intermittent, or persistent. In transient bacteremia, bacteria are present in the bloodstream for minutes to a few hours before being cleared from the body, and the result is typically harmless in healthy people. This can occur after manipulation of parts of the body normally colonized by bacteria, such as the mucosal surfaces of the mouth during teeth brushing, flossing, or dental procedures, or instrumentation of the bladder or colon. Intermittent bacteremia is characterized by periodic seeding of the same bacteria into the bloodstream by an existing infection elsewhere in the body, such as an abscess, pneumonia, or bone infection, followed by clearing of that bacteria from the bloodstream. This cycle will often repeat until the existing infection is successfully treated. Persistent bacteremia is characterized by the continuous presence of bacteria in the bloodstream. It is usually the result of an infected heart valve, a central line-associated bloodstream infection (CLABSI), an infected blood clot (suppurative thrombophlebitis), or an infected blood vessel graft. Persistent bacteremia can also occur as part of the infection process of typhoid fever, brucellosis, and bacterial meningitis. Left untreated, conditions causing persistent bacteremia can be potentially fatal.

Bacteremia is clinically distinct from sepsis, which is a condition where the blood stream infection is associated with an inflammatory response from the body, often causing abnormalities in body temperature, heart rate, breathing rate, blood pressure, and white blood cell count.

Cellulitis is caused by a type of bacteria entering the skin, usually by way of a cut, abrasion, or break in the skin. This break does not need to be visible. Group A "Streptococcus" and "Staphylococcus" are the most common of these bacteria, which are part of the normal flora of the skin, but normally cause no actual infection while on the skin's outer surface.

About 80% of cases of Ludwig's angina, or cellulitis of the submandibular space, are caused by dental infections. Mixed infections, due to both aerobes and anaerobes, are commonly associated with this type of cellulitis. Typically, this includes alpha-hemolytic streptococci, staphylococci, and bacteroides groups.

Predisposing conditions for cellulitis include insect or spider bite, blistering, animal bite, tattoos, pruritic (itchy) skin rash, recent surgery, athlete's foot, dry skin, eczema, injecting drugs (especially subcutaneous or intramuscular injection or where an attempted intravenous injection "misses" or blows the vein), pregnancy, diabetes, and obesity, which can affect circulation, as well as burns and boils, though debate exists as to whether minor foot lesions contribute. Occurrences of cellulitis may also be associated with the rare condition hidradenitis suppurativa or dissecting cellulitis.

The appearance of the skin assists a doctor in determining a diagnosis. A doctor may also suggest blood tests, a wound culture, or other tests to help rule out a blood clot deep in the veins of the legs. Cellulitis in the lower leg is characterized by signs and symptoms similar to those of a deep vein thrombosis, such as warmth, pain, and swelling (inflammation).

This reddened skin or rash may signal a deeper, more serious infection of the inner layers of skin. Once below the skin, the bacteria can spread rapidly, entering the lymph nodes and the bloodstream and spreading throughout the body. This can result in influenza-like symptoms with a high temperature and sweating or feeling very cold with shaking, as the sufferer cannot get warm.

In rare cases, the infection can spread to the deep layer of tissue called the fascial lining. Necrotizing fasciitis, also called by the media "flesh-eating bacteria", is an example of a deep-layer infection. It is a medical emergency.

Among the signs of subacute bacterial endocarditis are:

- Malaise

- Weakness

- Excessive sweat

- Fever

Cutaneous "Streptococcus iniae" infections cause a cellulitis of the hands, usually after a person handles tilapia, as this bacterium is a fish pathogen.

Affected individuals typically develop symptoms including high fevers, shaking, chills, fatigue, headaches, vomiting, and general illness within 48 hours of the initial infection. The erythematous skin lesion enlarges rapidly and has a sharply demarcated, raised edge. It appears as a red, swollen, warm, and painful rash, similar in consistency to an orange peel. More severe infections can result in vesicles (pox or insect bite-like marks), blisters, and petechiae (small purple or red spots), with possible skin necrosis (death). Lymph nodes may be swollen, and lymphedema may occur. Occasionally, a red streak extending to the lymph node can be seen.

The infection may occur on any part of the skin, including the face, arms, fingers, legs, and toes; it tends to favour the extremities. Fat tissue and facial areas, typically around the eyes, ears, and cheeks, are most susceptible to infection. Repeated infection of the extremities can lead to chronic swelling (lymphangitis).

Subacute bacterial endocarditis (also called endocarditis lenta) is a type of endocarditis (more specifically, infective endocarditis). Subacute bacterial endocarditis can be considered a form of type III hypersensitivity.

In most cases AIT is characterized by onset of pain, firmness, tenderness, redness or swelling in the anterior aspect of the neck. Patients will also present with a sudden fever, dysphagia and dysphonia. Symptoms may be present from 1 to 180 days, with most symptoms lasting an average of about 18 days. The main issue associated with the diagnosis of AIT is differentiating it from other more commonly seen forms of thyroid conditions. Pain, fever and swelling are often much more severe and continue to get worse in patients who have AIT compared to those with other thyroid conditions. Patients who are suspected of having AIT often undergo tests to detect for elevated levels of white blood cells as well as an ultrasound to reveal unilobular swelling. Depending on the age and immune status of the patient more invasive procedures may be performed such as fine needle aspiration of the neck mass to facilitate a diagnosis.

In cases where the infection is thought to be associated with a sinus fistula it is often necessary to confirm the presence of the fistula through surgery or laryngoscopic examination. While invasive procedures can often tell definitively whether or not a fistula is present, new studies are working on the use of computed tomography as a useful method to visualize and detect the presence of a sinus fistula.

An overwhelming post-splenectomy infection (OPSI) or Overwhelming post-splenectomy sepsis (OPSS) is a rare but rapidly fatal infection occurring in individuals following removal of the spleen. The infections are typically characterized by either meningitis or sepsis, and are caused by encapsulated organisms including "Streptococcus pneumoniae".

The risk of OPSI is 0.23–0.42 percent per year, with a lifetime risk of 5 percent. Most infections occur in the first few years following splenectomy, but the risk of OPSI is lifelong. OPSI is almost always fatal without treatment, and modern treatment has decreased the mortality to approximately 40–70 percent. Individuals with OPSI are most commonly treated with antibiotics and supportive care. Measures to prevent OPSI include vaccination and prophylactic antibiotics.

The signs and symptoms of Lemierre's syndrome vary, but usually start with a sore throat, fever, and general body weakness. These are followed by extreme lethargy, spiked fevers, rigors, swollen cervical lymph nodes, and a swollen, tender or painful neck. Often there is abdominal pain, diarrhea, nausea and vomiting during this phase. These signs and symptoms usually occur several days to 2 weeks after the initial symptoms.

Symptoms of pulmonary involvement can be shortness of breath, cough and painful breathing (pleuritic chest pain). Rarely, blood is coughed up. Painful or inflamed joints can occur when the joints are involved.

Septic shock can also arise. This presents with low blood pressure, increased heart rate, decreased urine output and an increased rate of breathing. Some cases will also present with meningitis, which will typically manifest as neck stiffness, headache and sensitivity of the eyes to light.

Liver enlargement and spleen enlargement can be found, but are not always associated with liver or spleen abscesses.

Other signs and symptoms that may occur:

- Headache (unrelated to meningitis)

- Memory loss

- Muscle pain

- Jaundice

- Decreased ability to open the jaw

- Crepitations are sometimes heard over the lungs

- Pericardial friction rubs as a sign of pericarditis (rare)

- Cranial nerve paralysis and Horner's syndrome (both rare)

Erysipelas is an acute infection typically with a skin rash, usually on any of the legs and toes, face, arms, and fingers. It is an infection of the upper dermis and superficial lymphatics, usually caused by beta-hemolytic group A "Streptococcus" bacteria on scratches or otherwise infected areas. Erysipelas is more superficial than cellulitis, and is typically more raised and demarcated. The term is from Greek ἐρυσίπελας, meaning "red skin".

Bacterial pneumonia is a type of pneumonia caused by bacterial infection.

Common signs and symptoms of orbital cellulitis include pain with eye movement, sudden vision loss, chemosis, bulging of the infected eye, and limited eye movement. Along with these symptoms, patients typically have redness and swelling of the eyelid, pain, discharge, inability to open the eye, occasional fever and lethargy. It is usually caused by a previous sinusitis. Other causes include infection of nearby structures, trauma and previous surgery.

Infective endocarditis is an infection of the inner surface of the heart, usually the valves. Symptoms may include fever, small areas of bleeding into the skin, heart murmur, feeling tired, and low red blood cells. Complications may include valvular insufficiency, heart failure, stroke, and kidney failure.

The cause is typically a bacterial infection and less commonly a fungal infection. Risk factors include valvular heart disease including rheumatic disease, congenital heart disease, artificial valves, hemodialysis, intravenous drug use, and electronic pacemakers. The bacterial most commonly involved are streptococci or staphylococci. Diagnosis is suspected based on symptoms and supported by blood cultures or ultrasound. There is also a noninfective form of endocarditis.

The usefulness of antibiotics following dental procedures for prevention is unclear. Some recommend them in those at high risk. Treatment is generally with intravenous antibiotics. The choice of antibiotics is based on the blood cultures. Occasionally heart surgery is required.

The number of people affected is about 5 per 100,000 per year. Rates, however, vary between regions of the world. Males are affected more often than females. The risk of death among those infected is about 25%. Without treatment it is almost universally fatal.