The elderly and those with a weakened immune system are especially vulnerable to contracting cellulitis. Diabetics are more susceptible to cellulitis than the general population because of impairment of the immune system; they are especially prone to cellulitis in the feet, because the disease causes impairment of blood circulation in the legs, leading to diabetic foot or foot ulcers. Poor control of blood glucose levels allows bacteria to grow more rapidly in the affected tissue, and facilitates rapid progression if the infection enters the bloodstream. Neural degeneration in diabetes means these ulcers may not be painful, thus often become infected. Those who have suffered poliomyelitis are also prone because of circulatory problems, especially in the legs.

Immunosuppressive drugs, and other illnesses or infections that weaken the immune system, are also factors that make infection more likely. Chickenpox and shingles often result in blisters that break open, providing a gap in the skin through which bacteria can enter. Lymphedema, which causes swelling on the arms and/or legs, can also put an individual at risk.

Diseases that affect blood circulation in the legs and feet, such as chronic venous insufficiency and varicose veins, are also risk factors for cellulitis.

Cellulitis is also common among dense populations sharing hygiene facilities and common living quarters, such as military installations, college dormitories, nursing homes, oil platforms, and homeless shelters.

The cause of juvenile cellulitis is unknown. Cytologic examination of aspirates of affected lymph nodes, pustules, abscesses, and joint fluid rarely reveal bacteria, and culture results of intact lesion are always negative for bacterial growth, suggesting a nonbacterial etiology. As signs resolve following treatment with glucocorticoids, the cause is likely to be an immune disorder.

People with recurrent boils are as well more likely to have a positive family history, take antibiotics, and to have been hospitalised, anemic, or diabetic; they are also more likely to have associated skin diseases and multiple lesions.

Other causes include poor immune system function such as from HIV/AIDS, diabetes, malnutrition, or alcoholism. Poor hygiene and obesity have also been linked. It may occur following antibiotic use due to the development of resistance to the antibiotics used. An associated skin disease favors recurrence. This may be attributed to the persistent colonization of abnormal skin with "S. aureus" strains, such as is the case in persons with atopic dermatitis.

Boils which recur under the arm, breast or in the groin area may be associated with hidradenitis suppurativa (HS).

Cellulitis in 2015 resulted in about 16,900 deaths worldwide, up from 12,600 in 2005.

Bacterial infections of the orbit have long been associated with a risk of catastrophic local

sequelae and intracranial spread.

The natural course of the disease, as documented by Gamble (1933), in the pre-antibiotic era,

resulted in death in 17% of patients and permanent blindness in 20%.

Large doses of glucocorticoids are the treatment of choice, and are administered until the signs have resolved. In uncomplicated cases, this can take up to a month. If dogs are not treated promptly and with high doses of steroids, severe scarring may occur. If there is evidence of secondary bacterial infection, treatment with antibiotics is required.

To prevent spread of impetigo to other people the skin and any open wounds should be kept clean. Care should be taken to keep fluids from an infected person away from the skin of a non-infected person. Washing hands, linens, and affected areas will lower the likelihood of contact with infected fluids. Sores should be covered with a bandage. Scratching can spread the sores; keeping nails short will reduce the chances of spreading. Infected people should avoid contact with others and eliminate sharing of clothing or linens.

Complications include hearing loss, blood infection, meningitis, cavernous sinus thrombosis, and optic nerve damage (which could lead to blindness).

Impetigo is more likely to infect children ages 2–5, especially those that attend school or day care. 70% of cases are the nonbullous form and 30% were the bullous form. Other factors can increase the risk of contracting impetigo such as diabetes mellitus, dermatitis, immunodeficiency disorders, and other irritable skin disorders. Impetigo occurs more frequently among people who live in warm climates.

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".

Tuberculous cellulitis is a skin condition resulting from infection with mycobacterium, and presenting as cellulitis.

Cutaneous group B streptococcal infection may result in orbital cellulitis or facial erysipelas in neonates.

Eosinophilic cellulitis, also known as Wells' syndrome, is a skin disease that presents with painful, red, raised, and warm patches of skin. The rash comes on suddenly, lasts for a few weeks, and often repeatedly comes back. Scar formation does not typically occur.

Eosinophilic cellulitis is of unknown cause. It is suspected to be an autoimmune disorder. It may be triggered by bites from insects such as spiders, fleas, or ticks, or from medications or surgery. Diagnosis is made after other potential cases are ruled out. Skin biopsy of the affected areas may show an increased number of eosinophils. Other conditions that may appear similar include cellulitis, contact dermatitis, and severe allergic reactions such as anaphylaxis.

Treatment is often with a corticosteroids. Steroids applied as a cream is generally recommended over the use of steroids by mouth. Antihistamines may be used to help with itchiness. Many times the condition goes away after a few weeks without treatment. The condition is uncommon with about 200 described cases. It affects both sexes with the same frequency. It was first described by George Crichton Wells in 1971.

Eosinophilic cellulitis is of unknown cause. It is suspected to be an autoimmune disorder. It may be triggered by bites from insects such as spiders, fleas, or ticks, or from medications or surgery.

Aeromonas infections may cause skin infections manifesting as cellulitis, pustules, and furuncles. An infection usually only causes mild infections of the skin but can also cause a more a serious infection called gastroenteritis?

Dissecting cellulitis of the scalp, also known as dissecting scalp cellulitis, dissecting folliculitis of the scalp, perifolliculitis capitis abscedens et suffodiens of Hoffman, perifolliculitis abscedens et suffodiens, or folliculitis abscedens et suffodiens, is an inflammatory condition of the scalp that can lead to scarring alopecia, which begins with deep inflammatory nodules, primarily over occiput, that progresses to coalescing regions of boggy scalp. Boggy tissue has a high fluid level that results in a spongy feeling.

Isotretinoin proves to be the medicine of choice for the treatment of the disease.

"Staphylococcus aureus", "Streptococcus pneumoniae", other streptococci, and anaerobes are the most common causes, depending on the origin of the infection.

The advent of the "Haemophilus influenzae" vaccine has dramatically decreased the incidence.

A variety of causes may lead to dacrocystitis. Most notably, obstruction of the nasolacrimal duct leads to stasis of the nasolacrimal fluid, which predisposes to infection. Staphylococcus aureus is a common bacterial pathogen causing infectious dacrocystitis. Sometimes, especially in women, stones may develop in the lacrimal gland, causing recurrent bouts of dacrocystitis; this condition is called "acute dacryocystic retention syndrome."

Also due to pneumococcus, infection due to surrounding structure such as paranasal sinuses.

Parasitic infestations, stings, and bites in humans are caused by several groups of organisms belonging to the following phyla: Annelida, Arthropoda, Bryozoa, Chordata, Cnidaria, Cyanobacteria, Echinodermata, Nemathelminthes, Platyhelminthes, and Protozoa.

The reason for its occurrence is unknown. Investigations have suspected that obesity and preceding psoriatic lesions cause local lymphatic disturbances, followed by the development of stasis papillomatosis. On the other hand, genetic or environmental factors may play a role. Some investigators have speculated that it represents an allergic response to an epidermal protein antigen created through increased hydrostatic pressure, whereas others believe that the skin has been compromised and is more susceptible to irritation and trauma.

The age is an important factor, because as some people get older the veins which carry blood from the legs back to the heart do not work as well as they use to. This causes fluid to settle in the lower legs.

The most important cause of this condition is insufficient lymphatic drainage, causing soft tissue swelling due to fluid accumulation. Obstruction of lymphatic tissue causes increased intravascular tissue protein; this will increase the production of fibroblasts and mast cells. Lymphatic obstruction due to any cause can increase the amount of proteins in the intravascular tissue, either by root osmotic pressure, or because it absorbs a little liquid. The further roteins increase the vascular fluid, fibroblasts and promote the ploriferation of mast cells which produce the clinical symptoms of nonpitting edema. The epidermis may be hyperkeratotic and warty and this predisposes to tissue cracks and allows secondary infection.

Dental infections account for approximately 80% of cases of Ludwig's angina. Mixed infections, due to both aerobes and anaerobes, are of the cellulitis associated with Ludwig's angina. Typically, these include alpha-hemolytic streptococci, staphylococci and bacteroides groups.

The route of infection in most cases is from infected lower molars or from pericoronitis, which is an infection of the gums surrounding the partially erupted lower (usually third) molars. Although the widespread involvement seen in Ludwig's usually develops in immunocompromised persons, it can also develop in otherwise healthy individuals. Thus, it is very important to obtain dental consultation for lower-third molars at the first sign of any pain, bleeding from the gums, sensitivity to heat/cold or swelling at the angle of the jaw.

There has been a single case reported where Ludwig's angina was thought to be caused by a recent Tongue piercing. In addition, Filipino boxer Pancho Villa (1901–1925) died after contracting Ludwig's Angina following a bout with Jimmy McLarnin.

Even without treatment they rarely result in death as they will naturally break through the skin.

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.

Antibiotics are aimed at gram positive bacteria. Medical attention should be sought if symptoms persist beyond 2–3 days.