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.

The most common primary sources of infection resulting in sepsis are the lungs, the abdomen, and the urinary tract. Typically, 50% of all sepsis cases start as an infection in the lungs. No definitive source is found in one third to one half of cases.

The increase in average age of the population, more people with chronic diseases, on immunosuppressive drugs, and increase in the number of invasive procedures being performed has led to an increased rate of sepsis.

Infections leading to sepsis usually are bacterial, but may be fungal or viral. Gram positive bacteria was the cause of sepsis before the introduction of antibiotics in the 1950s. After the introduction of antibiotics, gram negative bacteria became the predominant cause of sepsis from the 1960s to the 1980s. After 1980s, gram-positive bacteria, most commonly staphylococci, are thought to cause more than 50% of cases of sepsis. Other commonly implicated bacteria include "Streptococcus pyogenes", "Escherichia coli", "Pseudomonas aeruginosa", and "Klebsiella" species. Fungal sepsis accounts for approximately 5% of severe sepsis and septic shock cases; the most common cause of fungal sepsis is infection by "Candida" species of yeast, a nosocomial infection frequently acquired in hospitals.

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.

While there is tentative evidence for β-Blocker therapy to help control heart rate, evidence is not significant enough for its routine use. There is tentative evidence that steroids may be useful in improving outcomes.

Tentative evidence exists that Polymyxin B-immobilized fiber column hemoperfusion may be beneficial in treatment of septic shock. Trials are ongoing and it is currently being used in Japan and Western Europe.

Recombinant activated protein C (drotrecogin alpha) in a 2011 Cochrane review was found not to decrease mortality and to increase bleeding, and thus, was not recommended for use. Drotrecogin alfa (Xigris), was withdrawn from the market in October 2011.

There are several risk factors that increase the likelihood of developing bacteremia from any type of bacteria. These include:

- HIV infection

- Diabetes Mellitus

- Chronic hemodialysis

- Solid organ transplant

- Stem cell transplant

- Treatment with glucocorticoids

- Liver failure

When properly diagnosed, the mortality of Lemierre's syndrome is about 4.6%. Since this disease is not well known and often remains undiagnosed, mortality might be much higher.

Gram negative bacterial species are responsible for approximately 24% of all cases of healthcare-associated bacteremia and 45% of all cases of community-acquired bacteremia. In general, gram negative bacteria enter the bloodstream from infections in the respiratory tract, genitourinary tract, gastrointestinal tract, or hepatobiliary system. Gram-negative bacteremia occurs more frequently in elderly populations (65 years or older) and is associated with higher morbidity and mortality in this population.

"E.coli" is the most common cause of community-acquired bacteremia accounting for approximately 75% of cases. E.coli bacteremia is usually the result of a urinary tract infection. Other organisms that can cause community-acquired bacteremia include "pseudomonas aeruginosa", "klebsiella pneumoniae", and "proteus mirabilis". "Salmonella" infection, despite mainly only resulting in gastroenteritis in the developed world, is a common cause of bacteremia in Africa. It principally affects children who lack antibodies to Salmonella and HIV+ patients of all ages.

Among healthcare-associated cases of bacteremia, gram negative organisms are an important cause of bacteremia in the ICU. Catheters in the veins, arteries, or urinary tract can all create a way for gram negative bacteria to enter the bloodstream. Surgical procedures of the genitourinary tract, intestinal tract, or hepatobiliary tract can also lead to gram negative bacteremia. "Pseudomonas" and "enterobacter" species are the most important causes of gram negative bacteremia in the ICU.

A study performed at Strong Memorial Hospital in Rochester, New York, showed that infants ≤ 60 days old meeting the following criteria were at low-risk for having a serious bacterial illness:

- generally well-appearing

- previously healthy

- full term (at ≥37 weeks gestation)

- no antibiotics perinatally

- no unexplained hyperbilirubinemia that required treatment

- no antibiotics since discharge

- no hospitalizations

- no chronic illness

- discharged at the same time or before the mother

- no evidence of skin, soft tissue, bone, joint, or ear infection

- White blood cells (WBCs) count 5,000-15,000/mm

- absolute band count ≤ 1,500/mm

- urine WBC count ≤ 10 per high power field (hpf)

- stool WBC count ≤ 5 per high power field (hpf) "only in infants with diarrhea"

Those meeting these criteria likely do not require a lumbar puncture, and are felt to be safe for discharge home without antibiotic treatment, or with a single dose of intramuscular antibiotics, but will still require close outpatient follow-up.

One risk for Group B streptococcal infection (GBS) is Preterm rupture of membranes. Screening women for GBS (via vaginal and rectal swabbing) and treating culture positive women with intrapartum chemoprophylaxis is reducing the number of neonatal sepsis caused by GBS.

Inflammation can spread to other parts of the gut in patients with typhlitis. The condition can also cause the cecum to become distended and can cut off its blood supply. This and other factors can result in necrosis and perforation of the bowel, which can cause peritonitis and sepsis.

Historically, the mortality rate for typhlitis was as high as 50%, mostly because it is frequently associated with bowel perforation. More recent studies have demonstrated better outcomes with prompt medical management, generally with resolution of symptoms with neutrophil recovery without death

Lemierre's syndrome is currently rare, but was more common in the early 20th century before the discovery of penicillin. The reduced use of antibiotics for sore throats may have increased the risk of this disease, with 19 cases in 1997 and 34 cases in 1999 reported in the UK. The incidence rate is currently 0.8 cases per million in the general population, leading it to be termed the "forgotten disease". The disease is known to affect healthy young adults. The disease is becoming less rare with many cases being reported, however it is still known as "the forgotten disease" as many doctors are unaware of its existence, therefore often not even diagnosed which might considerably change the above-mentioned statistics. The mortality rate was 90% prior to antibiotic therapy, but is now generally quoted as 15% once this illness is correctly diagnosed and cured with proper medical treatment, although one series of cases reported mortality as low as 6.4%.

Multiple species of bacteria can be associated with the condition:

- Meningococcus is another term for the bacterial species "Neisseria meningitidis"; blood infection with said species usually underlies WFS. While many infectious agents can infect the adrenals, an acute, selective infection is usually meningococcus.

- "Pseudomonas aeruginosa" can also cause WFS.

- WFS can also be caused by "Streptococcus pneumoniae" infections, a common bacterial pathogen typically associated with meningitis in the adult and elderly population.

- "Mycobacterium tuberculosis" could also cause WFS. Tubercular invasion of the adrenal glands could cause hemorrhagic destruction of the glands and cause mineralocorticoid deficiency.

- "Staphylococcus aureus" has recently also been implicated in pediatric WFS.

- It can also be associated with "Haemophilus influenzae".

Viruses may also be implicated in adrenal problems:

- Cytomegalovirus can cause adrenal insufficiency, especially in the immunocompromised.

- Ebola virus infection may also cause similar acute adrenal failure.

Routine vaccination against meningococcus is recommended by the Centers for Disease Control and Prevention for all 11- to 18-year-olds and people who have poor splenic function (who, for example, have had their spleen removed or who have sickle-cell disease which damages the spleen), or who have certain immune disorders, such as a complement deficiency.

Note that, in neonates, sepsis is difficult to diagnose clinically. They may be relatively asymptomatic until hemodynamic and respiratory collapse is imminent, so, if there is even a remote suspicion of sepsis, they are frequently treated with antibiotics empirically until cultures are sufficiently proven to be negative. In addition to fluid resuscitation and supportive care, a common antibiotic regimen in infants with suspected sepsis is a beta-lactam antibiotic (usually ampicillin) in combination with an aminoglycoside (usually gentamicin) or a third-generation cephalosporin (usually cefotaxime—ceftriaxone is generally avoided in neonates due to the theoretical risk of kernicterus.) The organisms which are targeted are species that predominate in the female genitourinary tract and to which neonates are especially vulnerable to, specifically Group B Streptococcus, "Escherichia coli", and "Listeria monocytogenes" (This is the main rationale for using ampicillin versus other beta-lactams.) Of course, neonates are also vulnerable to other common pathogens that can cause meningitis and bacteremia such as "Streptococcus pneumoniae" and "Neisseria meningitidis". Although uncommon, if anaerobic species are suspected (such as in cases where necrotizing enterocolitis or intestinal perforation is a concern, clindamycin is often added.

Granulocyte-macrophage colony stimulating factor (GM-CSF) is sometimes used in neonatal sepsis. However, a 2009 study found that GM-CSF corrects neutropenia if present but it has no effect on reducing sepsis or improving survival.

Trials of probiotics for prevention of neonatal sepsis have generally been too small and statistically underpowered to detect any benefit, but a randomized controlled trial that enrolled 4,556 neonates in India reported that probiotics significantly reduced the risk of developing sepsis. The probiotic used in the trial was "Lactobacillus plantarum".

A very large meta-analysis investigated the effect of probiotics on preventing late-onset sepsis (LOS) in neonates. Probiotics were found to reduce the risk of LOS, but only in babies who were fed human milk exclusively. It is difficult to distinguish if the prevention was a result of the probiotic supplementation or if it was a result of the properties of human milk. It is also still unclear if probiotic administration reduces LOS risk in extremely low birth weight infants due to the limited number of studies that investigated it. Out of the 37 studies included in this systematic review, none indicated any safety problems related to the probiotics. It would be beneficial to clarify the relationship between probiotic supplementation and human milk for future studies in order to prevent late onset sepsis in neonates.

Septic shock is associated with significant mortality and is the leading non cardiac cause of death in intensive care units (ICUs).

The choice of fluids for resuscitation remains an area of research, the Surviving Sepsis Campaign an international consortium of experts, did not find adequate evidence to support the superiority crystalloid fluids versus colloid fluids. Drugs such as, pyridoxalated hemoglobin polyoxyethylene, which scavenge nitric oxide from the blood have been investigated. As well as methylene blue which may inhibit the nitric oxide-cyclic guanosine monophosphate (NO-cGMP) pathway which has been suggested to play a significant role in distributive shock.

Gas gangrene can cause myonecrosis (muscle tissue death), gas production, and sepsis. Progression to toxemia and shock is often very rapid. It can easily be noticed by the large, blackened sores that form, as well as a degree of loud and distinctive crepitus caused by gas escaping the necrotic tissue.

Typhlitis is a medical emergency and requires prompt management. Untreated typhlitis has a poor prognosis, particularly if associated with pneumatosis intestinalis (air in the bowel wall) and/or bowel perforation, and has significant morbidity unless promptly recognized and aggressively treated.

Successful treatment hinges on:

1. Early diagnosis provided by a high index of suspicion and the use of CT scanning

2. Nonoperative treatment for uncomplicated cases

3. Empiric antibiotics, particularly if the patient is neutropenic or at other risk of infection.

In rare cases of prolonged neutropenia and complications such as bowel perforation, neutrophil transfusions can be considered but have not been studied in a randomized control trial. Elective right hemicolectomy may be used to prevent recurrence but is generally not recommended

"...The authors have found nonoperative treatment highly effective in patients who do not manifest signs of peritonitis, perforation, gastrointestinal hemorrhage, or clinical deterioration. Recurrent typhlitis was frequent after conservative therapy (recurrence rate, 67 percent), however," as based on studies from the 1980s

The causes of SIRS are broadly classified as infectious or noninfectious. Causes of SIRS include:

- trauma

- burns

- pancreatitis

- ischemia

- hemorrhage

Other causes include:

- Complications of surgery

- Adrenal insufficiency

- Pulmonary embolism

- Complicated aortic aneurysm

- Cardiac tamponade

- Anaphylaxis

- Drug overdose

Gas gangrene (also known as clostridial myonecrosis and myonecrosis) is a bacterial infection that produces gas in tissues in gangrene. This deadly form of gangrene usually is caused by "Clostridium perfringens" bacteria. It is a medical emergency. About 1000 cases of gas gangrene occur yearly in the United States.

Myonecrosis is a condition of necrotic damage, specific to muscle tissue. It is often seen in infections with "C. perfringens" or any of myriad soil-borne anaerobic bacteria. Bacteria cause myonecrosis by specific exotoxins. These microorganisms are opportunistic and, in general, enter the body through significant skin breakage. Gangrenous infection by soil-borne bacteria was common in the combat injuries of soldiers well into the 20th century, because of nonsterile field surgery and the basic nature of care for severe projectile wounds.

Other causes of myonecrosis include envenomation by snakes of the "Bothrops" genus (family Viperidae), ischemic necrosis, caused by vascular blockage (e.g., diabetes type II), tumours that block or hoard blood supply, and disseminated intravascular coagulation or other thromboses.

The outlook is generally based on the severity of the infection. It is however a severe complication which may result in the death of the patient if the appropriate treatment is not administered. Patients are at risk of sepsis and multiple organ failure and in cases in which the infected abscess is not removed through surgery, the mortality rate can reach 100%.

Pancreatic abscesses usually develop in patients with pancreatic pseudocysts that become infected. They may also form as a result of fibrous wall formation around fluid collections or penetrating peptic ulcers. Other causes include gall stones or alcohol consumption and, in rare cases, drugs, blunt trauma and following extension abscess from nearby structures.

SIRS is frequently complicated by failure of one or more organs or organ systems. The complications of SIRS include:

- Acute lung injury

- Acute kidney injury

- Shock

- Multiple organ dysfunction syndrome

Causes (listed in order of decreasing frequency) include endometritis, urinary tract infection, pneumonia/atelectasis, wound infection, and septic pelvic thrombophlebitis. Septic risk factors for each condition are listed in order of the postpartum day (PPD) on which the condition generally occurs.

- PPD 0: atelectasis risk factors include general anesthesia, cigarette smoking, and obstructive lung disease.

- PPD 1–2: urinary tract infections risk factors include multiple catheterization during labor, multiple vaginal examinations during labor, and untreated bacteriuria.

- PPD 2–3: endometritis ( the most common cause ) risk factors include emergency cesarean section, prolonged membrane rupture, prolonged labor, and multiple vaginal examinations during labor.

- PPD 4–5: wound infection risk factors include emergency cesarean section, prolonged membrane rupture, prolonged labor, and multiple vaginal examination during labor.

- PPD 5–6: septic pelvic thrombophlebitis risk factors include emergency cesarean section, prolonged membrane rupture, prolonged labor, and diffuse difficult vaginal childbirth.

- PPD 7–21: mastitis risk factors include nipple trauma from breastfeeding.

Wet, or infected, gangrene is characterized by thriving bacteria and has a poor prognosis (compared to dry gangrene) due to sepsis resulting from the free communication between infected fluid and circulatory fluid. In wet gangrene, the tissue is infected by saprogenic microorganisms ("Clostridium perfringens" or "Bacillus fusiformis", for example), which cause tissue to swell and emit a smell. Wet gangrene usually develops rapidly due to blockage of venous (mainly) or arterial blood flow. The affected part is saturated with stagnant blood, which promotes the rapid growth of bacteria. The toxic products formed by bacteria are absorbed, causing systemic manifestation of sepsis and finally death. The affected part is edematous, soft, putrid, rotten, and dark.

Because of the high mortality associated with infected gangrene, an emergency salvage amputation, such as a guillotine amputation, is often needed to limit systemic effects of the infection. Such an amputation can be converted to a formal amputation, such as a below or above knee amputation.

Neutropenia is usually detected shortly after birth, affecting 6% to 8% of all newborns in neonatal intensive care units (NICUs). Out of the approximately 600,000 neonates annually treated in NICUs in the United States, 48,000 may be diagnosed as neutropenic. The incidence of neutropenia is greater in premature infants. Six to fifty-eight percent of preterm neonates are diagnosed with this auto-immune disease. The incidence of neutropenia correlates with decreasing birth weight. The disorder is seen up to 38% in infants that weigh less than 1000g, 13% in infants weighing less than 2500g, and 3% of term infants weighing more than 2500 g. Neutropenia is often temporary, affecting most newborns in only first few days after birth. In others, it becomes more severe and chronic indicating a deficiency in innate immunity.