Some strains of group A streptococci (GAS) cause severe infection. Severe infections are usually invasive, meaning that the bacteria has entered parts of the body where bacteria are not usually found, such as the blood, lungs, deep muscle or fat tissue. Those at greatest risk include children with chickenpox; persons with suppressed immune systems; burn victims; elderly persons with cellulitis, diabetes, vascular disease, or cancer; and persons taking steroid treatments or chemotherapy. Intravenous drug users also are at high risk. GAS is an important cause of puerperal fever worldwide, causing serious infection and, if not promptly diagnosed and treated, death in newly delivered mothers. Severe GAS disease may also occur in healthy persons with no known risk factors.

All severe GAS infections may lead to shock, multisystem organ failure, and death. Early recognition and treatment are critical. Diagnostic tests include blood counts and urinalysis as well as cultures of blood or fluid from a wound site.

Severe Group A streptococcal infections often occur sporadically but can be spread by person-to-person contact.

Public Health policies internationally reflect differing views of how the close contacts of people affected by severe Group A streptococcal infections should be treated. Health Canada and the US CDC recommend close contacts see their doctor for full evaluation and may require antibiotics; current UK Health Protection Agency guidance is that, for a number of reasons, close contacts should not receive antibiotics unless they are symptomatic but that they should receive information and advice to seek immediate medical attention if they develop symptoms. However, guidance is clearer in the case of mother-baby pairs: both mother and baby should be treated if either develops an invasive GAS infection within the first 28 days following birth (though some evidence suggests that this guidance is not routinely followed in the UK).

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.

A subset of children with acute, rapid-onset of tic disorders and obsessive compulsive disorder (OCD) are hypothesized to be due to an autoimmune response to group A beta-hemolytic streptococcal infection (PANDAS).

Horses may acquire cellulitis, usually secondarily to a wound (which can be extremely small and superficial) or to a deep-tissue infection, such as an abscess or infected bone, tendon sheath, or joint. Cellulitis from a superficial wound usually creates less lameness (grade 1–2 of 5) than that caused by septic arthritis (grade 4–5). The horse exhibits inflammatory edema, which is hot, painful swelling. This swelling differs from stocking up in that the horse does not display symmetrical swelling in two or four legs, but in only one leg. This swelling begins near the source of infection, but eventually continues down the leg. In some cases, the swelling also travels distally. Treatment includes cleaning the wound and caring for it properly, the administration of NSAIDs, such as phenylbutazone, cold hosing, applying a sweat wrap or a poultice, and mild exercise. Veterinarians may also prescribe antibiotics.

Cellulitis is also seen in staphylococcal and corynebacterial mixed infections in bulls.

"Other infections include:"

- "Closed-space infections of the fingertips, known as paronychia."

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.

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).

Hospital wards and, nurseries, and can be passed from person to person. Also many close contact sports. Therefore, it is advised that the patient has to try to limit as much human contact as possible to limit transmission of infection.

Staphylococcal scalded skin syndrome, (SSSS), also known as Pemphigus neonatorum or Ritter's disease, or Localized bullous impetigo is a dermatological condition caused by "Staphylococcus aureus".

With proper treatment, people usually recover in two to three weeks. The condition can, however, be fatal within hours.

Staphylococcal toxic shock syndrome is rare and the number of reported cases has declined significantly since the 1980s. Patrick Schlievert, who published a study on it in 2004, determined incidence at three to four out of 100,000 tampon users per year; the information supplied by manufacturers of sanitary products such as Tampax and Stayfree puts it at one to 17 of every 100,000 menstruating people per year.

Philip M. Tierno Jr., helped determine that tampons were behind TSS cases in the early 1980s. Tierno blames the introduction of higher-absorbency tampons in 1978. A study by Tierno also determined that all-cotton tampons were less likely to produce the conditions in which TSS can grow; this was done using a direct comparison of 20 brands of tampons including conventional cotton/rayon tampons and 100% organic cotton tampons from Natracare. In fact, Dr Tierno goes as far to state, "The bottom line is that you can get TSS with synthetic tampons, but not with an all-cotton tampon."

A rise in reported cases occurred in the early 2000s: eight deaths from the syndrome in California in 2002 after three successive years of four deaths per year, and Schlievert's study found cases in part of Minnesota more than tripled from 2000 to 2003. Schlievert considers earlier onset of menstruation to be a cause of the rise; others, such as Philip M. Tierno and Bruce A. Hanna, blame new high-absorbency tampons introduced in 1999 and manufacturers discontinuing warnings not to leave tampons in overnight.

After 48 hours the disease is considered no longer contagious assuming the proper antibiotic treatments have been administered.

The prognosis of SSSS in children is excellent, with complete resolution within 10 days of treatment, and without significant scarring. However, SSSS must be differentiated carefully from toxic epidermal necrolysis, which carries a poor prognosis. The prognosis in adults is generally much worse, and depends upon various factors such as time to treatment, host immunity, and comorbidities.

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.

Approximately 20–35% of people with severe sepsis and 30–70% of people with septic shock die. Lactate is a useful method of determining prognosis with those who have a level greater than 4 mmol/L having a mortality of 40% and those with a level of less than 2 mmol/L have a mortality of less than 15%.

There are a number of prognostic stratification systems such as APACHE II and Mortality in Emergency Department Sepsis. APACHE II factors in the person's age, underlying condition, and various physiologic variables to yield estimates of the risk of dying of severe sepsis. Of the individual covariates, the severity of underlying disease most strongly influences the risk of death. Septic shock is also a strong predictor of short- and long-term mortality. Case-fatality rates are similar for culture-positive and culture-negative severe sepsis. The Mortality in Emergency Department Sepsis (MEDS) score is simpler and useful in the emergency department environment.

Some people may experience severe long-term cognitive decline following an episode of severe sepsis, but the absence of baseline neuropsychological data in most people with sepsis makes the incidence of this difficult to quantify or to study.

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.

Sepsis causes millions of deaths globally each year and is the most common cause of death in people who have been hospitalized. The worldwide incidence of sepsis is estimated to be 18 million cases per year. In the United States sepsis affects approximately 3 in 1,000 people, and severe sepsis contributes to more than 200,000 deaths per year.

Sepsis occurs in 1–2% of all hospitalizations and accounts for as much as 25% of ICU bed utilization. Due to it rarely being reported as a primary diagnosis (often being a complication of cancer or other illness), the incidence, mortality, and morbidity rates of sepsis are likely underestimated. A study by the Agency for Healthcare Research and Quality (AHRQ) of selected States found that there were approximately 651 hospital stays per 100,000 population with a sepsis diagnosis in 2010. It is the second-leading cause of death in non-coronary intensive care unit (ICU) and the tenth-most-common cause of death overall (the first being heart disease). Children under 12 months of age and elderly people have the highest incidence of severe sepsis. Among U.S. patients who had multiple sepsis hospital admissions in 2010, those who were discharged to a skilled nursing facility or long term care following the initial hospitalization were more likely to be readmitted than those discharged to another form of care. A study of 18 U.S. States found that, amongst Medicare patients in 2011, sepsis was the second most common principal reason for readmission within 30 days.

Several medical conditions increase a person's susceptibility to infection and developing sepsis. Common sepsis risk factors include age (especially the very young and old); conditions that weaken the immune system such as cancer, diabetes, or the absence of a spleen; and major trauma and burns.

The cause of this disease is "Yersinia pseudotuberculosis" serotype O1. 95% are subtype O1b.

"Yersinia pseudotuberculosis" has been divided into 6 genetic groups: group 1 has only been isolated from the Far East.

A study conducted in November of 2017, conveyed a correlation between blepharitis and early onset metabolic syndrome (MetS). To investigate the relationship between blepharitis and MetS, researchers used the Longitudinal Health Insurance Database in Taiwan. Results indicated that hyperlipidaemia and coronary artery disease were significantly correlated with the prior development of blepharitis. Therefore, blepharitis was shown to be significantly related to MetS and can serve as an early indication of the condition.

In another recent study, the presence of Demodex has been unveiled as a common cause of blepharitis. However, the pathogenesis of demodicosis is still unclear. In this study, researchers provide a diagnosis of the disease and propose diagnostic criteria of Demodex blepharitis.

If ehrlichiosis is suspected, treatment should not be delayed while waiting for a definitive laboratory confirmation, as prompt doxycycline therapy has been associated with improved outcomes. Doxycycline is the treatment of choice.

Presentation during early pregnancy can complicate treatment.Rifampin has been used in pregnancy and in patients allergic to doxycycline.

Sepsis has a worldwide incidence of more than 20 million cases a year, with mortality due to septic shock reaching up to 50 percent even in industrialized countries.

According to the U.S. Centers for Disease Control, septic shock is the thirteenth leading cause of death in the United States and the most frequent cause of deaths in intensive care units. There has been an increase in the rate of septic shock deaths in recent decades, which is attributed to an increase in invasive medical devices and procedures, increases in immunocompromised patients, and an overall increase in elderly patients.

Tertiary care centers (such as hospice care facilities) have 2-4 times the rate of bacteremia than primary care centers, 75% of which are hospital-acquired infections.

The process of infection by bacteria or fungi may result in systemic signs and symptoms that are variously described. Approximately 70% of septic shock cases were once traceable to gram-negative bacteria that produce endotoxins, however, with the emergence of MRSA and the increased use of arterial and venous catheters, gram-positive bacteria are implicated approximately as commonly as bacilli. In rough order of increasing severity these are, bacteremia or fungemia; sepsis, severe sepsis or sepsis syndrome; septic shock, refractory septic shock, multiple organ dysfunction syndrome, and death.

35% of septic shock cases derive from urinary tract infections, 15% from the respiratory tract, 15% from skin catheters (such as IVs), and more than 30% of all cases are idiopathic in origin.

The mortality rate from sepsis is approximately 40% in adults and 25% in children. It is significantly greater when sepsis is left untreated for more than seven days.

Far East scarlet-like fever or scarlatinoid fever is an infectious disease caused by the gram negative bacillus "Yersinia pseudotuberculosis". In Japan it is called Izumi fever.

Staphylococcal enteritis is an inflammation that is usually caused by eating or drinking substances contaminated with staph enterotoxin. The toxin, not the bacterium, settles in the small intestine and causes inflammation and swelling. This in turn can cause abdominal pain, cramping, dehydration, diarrhea and fever.

"Staphylococcus aureus" is a Gram-positive, facultative anaerobe, coccal (round shaped) bacteria that appears in grape-like clusters that can thrive in high salt and low water activity habitats. "S. aureus" bacteria can live on the skin which is one of the primary modes of transmission. "S. aureus" can cause a range of illnesses from minor skin infections to Staphylococcus aureus food poisoning enteritis. Since humans are the primary source, cross-contamination is the most common way the microorganism is introduced into foods. Foods at high risks are those prepared in large quantities.

Staphylococcus aureus is a true food poisoning organism. It produces a heat stable enterotoxin when allowed to grow for several hours in foods such as cream-filled baked goods, poultry meat, gravies, eggs, meat salads, puddings and vegetables. It is important to note that the toxins may be present in dangerous amounts in foods that have no signs of spoilage, such as a bad smell, any off color, odor, or textural or flavor change.

Enteritis is the inflammation of the small intestine. It is generally caused by eating or drinking substances that are contaminated with bacteria or viruses. The bacterium and/or toxin settles in the small intestine and cause inflammation and swelling. This in turn can cause abdominal pain, cramping, diarrhea, fever, and dehydration. There are other types of enteritis, the types include: bacterial gastroenteritis, "Campylobacter" enteritis, "E. coli" enteritis, radiation enteritis, "Salmonella" enteritis and "Shigella" enteritis.

"S. aureus" is an enterotoxin producer. Enterotoxins are chromosomally encoded exotoxins that are produced and secreted from several bacterial organisms. It is a heat stable toxin and is resistant to digestive protease. It is the ingestion of the toxin that causes the inflammation and swelling of the intestine.