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.

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.

In addition to symptoms related to the provoking cause, sepsis is frequently associated with either fever, low body temperature, rapid breathing, elevated heart rate, confusion, and edema. Early signs are a rapid heart rate, decreased urination, and high blood sugar. Signs of established sepsis include confusion, metabolic acidosis (which may be accompanied by faster breathing and lead to a respiratory alkalosis), low blood pressure due to decreased systemic vascular resistance, higher cardiac output, and dysfunctions of blood coagulation (where clotting may lead to organ failure).

The drop in blood pressure seen in sepsis may lead to shock. This may result in light-headedness. Bruising or intense bleeding may occur.

The differential diagnosis for sepsis is broad and has to examine (to exclude) the noninfectious conditions that may cause the systemic signs of SIRS: alcohol withdrawal, acute pancreatitis, burns, pulmonary embolism, thyrotoxicosis, anaphylaxis, adrenal insufficiency, and neurogenic shock. Hyperinflammatory syndromes such as hemophagocytic lymphohistiocytosis (HLH) may have similar symptoms and should also be included in differential diagnosis.

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.

Orbital cellulitis

is inflammation of eye tissues behind the orbital septum. It most commonly refers to an acute spread of infection into the eye socket from either the adjacent sinuses or through the blood. It may also occur after trauma.

When it affects the rear of the eye, it is known as retro-orbital cellulitis.

It should not be confused with periorbital cellulitis, which refers to cellulitis anterior to the septum.

The main coagulase-positive staphylococcus is Staphylococcus aureus, although not all strains of Staphylococcus aureus are coagulase positive. These bacteria can survive on dry surfaces, increasing the chance of transmission. S. aureus is also implicated in toxic shock syndrome; during the 1980s some tampons allowed the rapid growth of S. aureus, which released toxins that were absorbed into the bloodstream. Any S. aureus infection can cause the staphylococcal scalded skin syndrome, a cutaneous reaction to exotoxin absorbed into the bloodstream. It can also cause a type of septicaemia called pyaemia. The infection can be life-threatening. Problematically, Methicillin-resistant Staphylococcus aureus (MRSA) has become a major cause of hospital-acquired infections, and is being, MRSA has also been recognized with increasing frequency in community-acquired infections. The symptoms of a Staph Infection include a collection of pus, such as a boil or furuncle, or abscess. The area is typically tender or painful and may be reddened or swollen.

The generic name Staphylococcus is derived from the Greek word "staphyle," meaning bunch of grapes, and "kokkos," meaning granule. The bacteria, when seen under a microscope, appear like a branch of grapes or berries.

Infection with "Y. enterocolitica" can cause a variety of symptoms depending on the age of the person infected, therefore it's often referred to as "monkey of diseases". Common symptoms in children are fever, abdominal pain, and diarrhea, which is often bloody. Symptoms typically develop 4 to 7 days after exposure and may last 1 to 3 weeks or longer. In older children and adults, right-sided abdominal pain and fever may be the predominant symptoms, and may be confused with appendicitis. In a small proportion of cases, complications such as skin rash, joint pains, ileitis, erythema nodosum, and sometimes septicemia, acute arthritis or the spread of bacteria to the bloodstream (bacteremia) can occur.

Signs and symptoms usually include a fever greater than , chills, low abdominal pain, and possibly bad smelling vaginal discharge.

Infective endocarditis may also be classified as "culture-positive" or "culture-negative". By far the most common cause of a "culture-negative" endocarditis is prior administration of antibiotics.

Sometimes microorganisms can take a longer period of time to grow in the culture media, such organisms are said to be "fastidious" because they have demanding growth requirements. Some examples include pathogens like "Aspergillus" species, "Brucella" species, "Coxiella burnetii", "Chlamydia" species, and HACEK bacteria. Due to delay in growth and identification in these cases, patients may be erroneously classified as "culture-negative" endocarditis.

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.

After childbirth a woman's genital tract has a large bare surface, which is prone to infection. Infection may be limited to the cavity and wall of her uterus, or it may spread beyond to cause septicaemia (blood poisoning) or other illnesses, especially when her resistance has been lowered by a long labour or severe bleeding. Puerperal infection is most common on the raw surface of the interior of the uterus after separation of the placenta (afterbirth); but pathogenic organisms may also affect lacerations of any part of the genital tract. By whatever portal, they can invade the bloodstream and lymph system to cause septicemia, cellulitis (inflammation of connective tissue), and pelvic or generalized peritonitis (inflammation of the abdominal lining). The severity of the illness depends on the virulence of the infecting organism, the resistance of the invaded tissues, and the general health of the woman. Organisms commonly producing this infection are "Streptococcus pyogenes"; staphylococci (inhabitants of the skin and of pimples, carbuncles, and many other pustular eruptions); the anaerobic streptococci, which flourish in devitalized tissues such as may be present after long and injurious labour and unskilled instrumental delivery; "Escherichia coli" and "Clostridium perfringens" (inhabitants of the lower bowel); and "Clostridium tetani".

The following is a list of common signs and symptoms found with neonatal meningitis.

- Fever

- poor appetite

- anterior fontanelle bulging

- seizure

- jitteriness

- dyspnea

- irritability

- anorexia

- vomiting

- diarrhea

- abdominal distention (increase in abdominal size)

- neck rigidity

- cyanosis

- jaundice

- and sunset eyes (downward gaze of the eyes)

- abnormal body temperature (hypo-or hyperthermia)

- change of activity (lethargy or irritability)

Unfortunately these symptoms are unspecific and may point to many different conditions.

Laboratory features that are characteristic of neonatal bacterial meningitis include:

- Isolation of a bacterial pathogen from the cerebrospinal fluid (CSF) by culture and/or visualization by Gram stain

- Increased CSF white blood cell (WBC) count (typically >1000 WBC/mL, but may be lower, especially with gram-positive organisms) with a predominance of neutrophils

- Elevated CSF protein concentration (>150 mg/dL in preterm (premature birth) and >100 mg/dL in term (on time) infants)

- Decreased CSF glucose concentration (<20 mg/dL [1.1 mmol/L] in preterm (premature birth) and <30 mg/dL [1.7 mmol/L] in term (on time) infants)

The symptoms of an infection depend on the type of disease. Some signs of infection affect the whole body generally, such as fatigue, loss of appetite, weight loss, fevers, night sweats, chills, aches and pains. Others are specific to individual body parts, such as skin rashes, coughing, or a runny nose.

In certain cases, infectious diseases may be asymptomatic for much or even all of their course in a given host. In the latter case, the disease may only be defined as a "disease" (which by definition means an illness) in hosts who secondarily become ill after contact with an asymptomatic carrier. An infection is not synonymous with an infectious disease, as some infections do not cause illness in a host.

Yersiniosis is an infectious disease caused by a bacterium of the genus "Yersinia". In the United States, most yersiniosis infections among humans are caused by "Yersinia enterocolitica". The infection by "Y. enterocolitica" is also known as pseudotuberculosis. Yersiniosis is mentioned as a specific zoonotic disease to prevent outbreaks in European Council Directive 92/117/EEC.

Infection with " Y . enterocolitica" occurs most often in young children. The infection is thought to be contracted through the consumption of undercooked meat products, unpasteurized milk, or water contaminated by the bacteria. It has been also sometimes associated with handling raw chitterlings.

Another bacterium of the same genus, "Yersinia pestis", is the cause of Plague.

The first symptom is a sudden onset of swelling of the face, which develops within two days into papules and pustues on the lips, nose, and around the eyes. These pustules release a purulent discharge, causing a crust to form on the skin. There is also lymphadenopathy (swelling of lymph nodes) in the main lymph nodes of the head. The feet, body, prepuce or perianal area may be affected. Pustular otitis externa can occur, with the pinnae (ear flaps) becoming thickened by edema. Affected areas are painful but not itchy. Approximately half of affected puppies have lethargy and depression. Less common symptoms include pyrexia, anorexia, and joint pain, caused by sterile suppurative arthritis.

Toxic shock syndrome (TSS) is a condition caused by bacterial toxins. Symptoms may include fever, rash, skin peeling, and low blood pressure. There may also be symptoms related to the specific underlying infection such as mastitis, osteomyelitis, necrotising fasciitis, or pneumonia.

TSS is caused by bacteria of either the "Streptococcus pyogenes" or "Staphylococcus aureus" type. Streptococcal toxic shock syndrome (STSS) is sometimes referred to as toxic shock-like syndrome (TSLS). The underlying mechanism involves the production of superantigens during an invasive streptococcus infection or a localized staphylococcus infection. Risk factors for the staphylococcal type include the use of very absorbent tampons and skin lesions in young children. Diagnosis is typically based on symptoms.

Treatment includes antibiotics, incision and drainage of any abscesses, and possibly intravenous immunoglobulin. The need for rapid removal of infected tissue via surgery in those with a streptococcal cause while commonly recommended is poorly supported by the evidence. Some recommend delaying surgical debridement. The overall risk of death in streptococcal disease is about 50% while in staphylococcal disease it is around 5%. Death may occur within 2 days.

In the United States streptococcal TSS occurs in about 3 per 100,000 per year while staphylococcal TSS occurs in about 0.5 per 100,000 per year. The condition is more common in the developing world. It was first described in 1927. Due to the association with very absorbent tampons, these products were removed from sale.

Infections can be classified by the anatomic location or organ system infected, including:

- Urinary tract infection

- Skin infection

- Respiratory tract infection

- Odontogenic infection (an infection that originates within a tooth or in the closely surrounding tissues)

- Vaginal infections

- Intra-amniotic infection

In addition, locations of inflammation where infection is the most common cause include pneumonia, meningitis and salpingitis.

Symptoms of toxic shock syndrome vary depending on the underlying cause. TSS resulting from infection with the bacterium "Staphylococcus aureus" typically manifests in otherwise healthy individuals via signs and symptoms including high fever, accompanied by low blood pressure, malaise and confusion, which can rapidly progress to stupor, coma, and multiple organ failure. The characteristic rash, often seen early in the course of illness, resembles a sunburn, and can involve any region of the body including the lips, mouth, eyes, palms and soles. In patients who survive the initial phase of the infection, the rash desquamates, or peels off, after 10–14 days.

In contrast, TSS caused by the bacterium "Streptococcus pyogenes", or TSLS, typically presents in people with pre-existing skin infections with the bacteria. These individuals often experience severe pain at the site of the skin infection, followed by rapid progression of symptoms as described above for TSS. In contrast to TSS caused by "Staphylococcus", streptococcal TSS less often involves a sunburn-like rash.

A hospital-acquired infection (HAI), also known as a nosocomial infection, is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is sometimes instead called a health care–associated infection (HAI or HCAI). Such an infection can be acquired in hospital, nursing home, rehabilitation facility, outpatient clinic, or other clinical settings. Infection is spread to the susceptible patient in the clinical setting by various means. Health care staff can spread infection, in addition to contaminated equipment, bed linens, or air droplets. The infection can originate from the outside environment, another infected patient, staff that may be infected, or in some cases, the source of the infection cannot be determined. In some cases the microorganism originates from the patient's own skin microbiota, becoming opportunistic after surgery or other procedures that compromise the protective skin barrier. Though the patient may have contracted the infection from their own skin, the infection is still considered nosocomial since it develops in the health care setting.

In the United States, the Centers for Disease Control and Prevention estimated roughly 1.7 million hospital-associated infections, from all types of microorganisms, including bacteria and fungi combined, cause or contribute to 99,000 deaths each year. In Europe, where hospital surveys have been conducted, the category of gram-negative infections are estimated to account for two-thirds of the 25,000 deaths each year. Nosocomial infections can cause severe pneumonia and infections of the urinary tract, bloodstream and other parts of the body. Many types are difficult to treat with antibiotics. In addition, antibiotic resistance can complicate treatment.

Common symptoms of "Staphylococcus aureus" food poisoning include: a rapid onset which is usually 1–6 hours, nausea, explosive vomiting for up to 24 hours, abdominal cramps/pain, headache, weakness, diarrhea and usually a subnormal body temperature. Symptoms usually start one to six hours after eating and last less than 12 hours. The duration of some cases may take two or more days to fully resolve.

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.

The primary skin lesion usually starts with a macule that is painless, round and erythematous. Then, it develops into a pustule, and then a bulla with central hemorrhagic focus. The bullae progresses into an ulcer which extends laterally. Finally it becomes a gangrenous ulcer with central black eschar surrounded by erythematous halo.

The lesion may be single or multiple. They are most commonly seen in perineum and under arm pit. However, it can occur in any part of the body.