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.

Bacteremia is typically transient and is quickly removed from the blood by the immune system.

Bacteremia frequently evokes a response from the immune system called Sepsis, which consists of symptoms such as fever, chills, and hypotension. Severe immune responses to bacteremia may result in septic shock and "multiple organ dysfunction syndrome", which are potentially fatal.

Bacteremia can travel through the blood stream to distant sites in the body and cause infection (hematogenous spread). Hematogenous spread of bacteria is part of the pathophysiology of certain infections of the heart (endocarditis), structures around the brain (meningitis), and tuberculosis of the spine (Pott's disease). Hematogenous spread of bacteria is responsible for many bone infections (osteomyelitis).

Prosthetic cardiac implants (for example artificial heart valves) are especially vulnerable to infection from bacteremia.

Prior to widespread use of vaccines, occult bacteremia was an important consideration in febrile children that appeared otherwise well.

Signs and symptoms of spontaneous bacterial peritonitis include fevers, chills, nausea, vomiting, abdominal tenderness, and general malaise. Affected individuals may complain of abdominal pain and worsening ascites. Thirteen percent of patients have no signs or symptoms. Hepatic encephalopathy may be the only manifestation of SBP; in the absence of a clear precipitant for the encephalopathy, all patients should undergo paracentesis, or sampling of the ascites fluid, in order to assess for SBP.

Bacillary dysentery is a type of dysentery, and is a severe form of shigellosis.

Bacillary dysentery is associated with species of bacteria from the Enterobacteriaceae family. The term is usually restricted to "Shigella" infections.

Shigellosis is caused by one of several types of "Shigella" bacteria. Three species are associated with bacillary dysentery: "Shigella sonnei, Shigella flexneri" and "Shigella dysenteriae". A study in China indicated that "Shigella flexneri" 2a was the most common serotype.

Salmonellosis caused by "Salmonella enterica" (serovar "Typhimurium") has also been described as a cause of bacillary dysentery, though this definition is less common. It is sometimes listed as an explicit differential diagnosis of bacillary dysentery, as opposed to a cause.

Bacillary dysentery should not be confused with diarrhea caused by other bacterial infections. One characteristic of bacillary dysentery is blood in stool, which is the result of invasion of the mucosa by the pathogen.

Spontaneous bacterial peritonitis (SBP) is the development of a bacterial infection in the peritoneum causing peritonitis, despite the absence of an obvious source for the infection. It occurs almost exclusively in people with portal hypertension (increased pressure over the portal vein), usually as a result of cirrhosis of the liver. It can also occur in patients with nephrotic syndrome.

The diagnosis of SBP requires paracentesis (aspiration of fluid with a needle) from the abdominal cavity. If the fluid contains bacteria or large numbers of neutrophil granulocytes (>250 cells/µL) (a type of white blood cells), infection is confirmed and antibiotics are required to avoid complications. In addition to antibiotics, infusions of albumin are usually administered.

Transmission is fecal-oral and is remarkable for the small number of organisms that may cause disease (10 ingested organisms cause illness in 10% of volunteers, and 500 organisms cause disease in 50% of volunteers). "Shigella" bacteria invade the intestinal mucosal cells but do not usually go beyond the lamina propria. Dysentery is caused when the bacteria escape the epithelial cell phagolysosome, multiply within the cytoplasm, and destroy host cells. Shiga toxin causes hemorrhagic colitis and hemolytic-uremic syndrome by damaging endothelial cells in the microvasculature of the colon and the glomeruli, respectively. In addition, chronic arthritis secondary to "S. flexneri" infection, called reactive arthritis, may be caused by a bacterial antigen; the occurrence of this syndrome is strongly linked to HLA-B27 genotype, but the immunologic basis of this reaction is not understood.

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.

After a short incubation period of a few hours to one day, the bacteria multiply in the intestinal lumen, causing an intestinal inflammation. Most people with salmonellosis develop diarrhea, fever, vomiting, and abdominal cramps 12 to 72 hours after infection. Diarrhea is often mucopurulent (containing mucus or pus) and bloody. In most cases, the illness lasts four to seven days, and most people recover without treatment. In some cases, though, the diarrhea may be so severe that the patient becomes dangerously dehydrated and must be taken to a hospital. At the hospital, the patient may receive intravenous fluids to treat the dehydration, and may be given medications to provide symptomatic relief, such as fever reduction. In severe cases, the "Salmonella" infection may spread from the intestines to the blood stream, and then to other body sites, and can cause death, unless the person is treated promptly with antibiotics.

In otherwise healthy adults, the symptoms can be mild. Normally, no sepsis occurs, but it can occur exceptionally as a complication in the immunocompromised. However, in people at risk such as infants, small children, and the elderly, "Salmonella" infections can become very serious, leading to complications. In infants, dehydration can cause a state of severe toxicity. Extraintestinal localizations are possible, especially "Salmonella" meningitis in children, osteitis, etc. Children with sickle-cell anemia who are infected with "Salmonella" may develop osteomyelitis. Treatment of osteomyelitis, in this case, will be to use fluoroquinolones (ciprofloxacin, levofloxacin, etc., and nalidixic acid).

Those whose only symptom is diarrhea usually completely recover, but their bowel habits may not return to normal for several months.

Typhoid fever occurs when "Salmonella" bacteria enter the lymphatic system and cause a systemic form of salmonellosis. Endotoxins first act on the vascular and nervous apparatus, resulting in increased permeability and decreased tone of the vessels, upset thermal regulation, vomiting, and diarrhea. In severe forms of the disease, enough liquid and electrolytes are lost to upset the water-salt metabolism, decrease the circulating blood volume and arterial pressure, and cause hypovolemic shock. Septic shock may also develop. Shock of mixed character (with signs of both hypovolemic and septic shock) are more common in severe salmonellosis. Oliguria and azotemia develop in severe cases as a result of renal involvement due to hypoxia and toxemia.

Signs and symptoms may range from mild abdominal discomfort to full-blown dysentery characterized by cramps, diarrhea, with slimy-consistent stools, fever, blood, pus, or mucus in stools or tenesmus. Onset time is 12 to 96 hours, and recovery takes 5 to 7 days.

Infections are associated with mucosal ulceration, rectal bleeding, and drastic dehydration. Reactive arthritis and hemolytic uremic syndrome are possible sequelae that have been reported in the aftermath of shigellosis.

"Shigella" can be transmitted through food, including salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk and dairy products, and meat. Contamination of these foods is usually through the fecal-oral route. Fecally contaminated water and unsanitary handling by food handlers are the most common causes of contamination. Apart from hand-to-mouth infection, shigellosis is transmitted through fomites, water and mechanical vectors like houseflies.

The most common neurological symptom includes seizures.

Specific types of enterocolitis include:

- necrotizing enterocolitis (most common in premature infants)

- pseudomembranous enterocolitis (also called "Pseudomembranous colitis")

Shigellosis is a type of diarrhea caused by a bacterial infection with "Shigella". Symptoms generally start one to two days after exposure and include diarrhea, fever, abdominal pain, and pain with passing stool. Blood may be present in the stool. Symptoms typically last five to seven days. Complications can include post infectious arthritis, sepsis, seizures, and hemolytic uremic syndrome.

Shigellosis is caused by four specific types of "Shigella". These are typically spread by exposure to infected feces. This can occur via contaminated food, water, or hands. Contamination may be spread by flies or when changing diapers. Diagnosis is by stool culture.

Prevention is by properly washing the hands. There is no vaccine. Shigellosis usually resolves without specific treatment. Sufficient fluids by mouth and rest is recommended. Bismuth subsalicylate may help with the symptoms; however, medications that slow the bowels such as loperamide are not recommended. In severe cases antibiotics may be used but resistance is common. Commonly used antibiotics include ciprofloxacin or azithromycin.

Globally shigellosis occurs in at least 80 million people and results in about 700,000 deaths a year. Most cases occur in the developing world. Young children are most commonly affected. Outbreaks of disease may occur in childcare settings and schools. It is also relatively common among travelers. In the United States about half a million cases occur a year.

Enterocolitis or coloenteritis is an inflammation of the digestive tract, involving enteritis of the small intestine and colitis of the colon. It may be caused by various infections, with bacteria, viruses, fungi, parasites, or other causes. Common clinical manifestations of enterocolitis are frequent diarrheal defecations, with or without nausea, vomiting, abdominal pain, fever, chills, alteration of general condition. General manifestations are given by the dissemination of the infectious agent or its toxins throughout the body, or – most frequently – by significant losses of water and minerals, the consequence of diarrhea and vomiting.

Among the causal agents of acute enterocolitis are:

- bacteria: "Salmonella", "Shigella", "Escherichia coli", "Campylobacter" etc.;

- viruses: enteroviruses, rotaviruses, Norwalk virus, adenoviruses;

- fungi: candidiasis, especially in immunosuppressed patients or who have previously received prolonged antibiotic treatment;

- parasites: "Giardia lamblia" (with high frequency of infestation in the population, but not always with clinical manifestations), "Balantidium coli", "Blastocystis homnis", "Cryptosporidium" (diarrhea in people with immunosuppression), "Entamoeba histolytica" (produces the amebian dysentery, common in tropical areas).

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.

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.

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.

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.

Gastroenteritis typically involves both diarrhea and vomiting, or less commonly, presents with only one or the other. Abdominal cramping may also be present. Signs and symptoms usually begin 12–72 hours after contracting the infectious agent. If due to a viral agent, the condition usually resolves within one week. Some viral causes may also be associated with fever, fatigue, headache, and muscle pain. If the stool is bloody, the cause is less likely to be viral and more likely to be bacterial. Some bacterial infections may be associated with severe abdominal pain and may persist for several weeks.

Children infected with rotavirus usually make a full recovery within three to eight days. However, in poor countries treatment for severe infections is often out of reach and persistent diarrhea is common. Dehydration is a common complication of diarrhea, and a child with a significant degree of dehydration may have a prolonged capillary refill, poor skin turgor, and abnormal breathing. Repeat infections are typically seen in areas with poor sanitation, and malnutrition, stunted growth, and long-term cognitive delays can result.

Reactive arthritis occurs in 1% of people following infections with "Campylobacter" species, and Guillain–Barré syndrome occurs in 0.1%. Hemolytic uremic syndrome (HUS) may occur due to infection with Shiga toxin-producing "Escherichia coli" or "Shigella" species, causing low platelet counts, poor kidney function, and low red blood cell count (due to their breakdown). Children are more predisposed to getting HUS than adults. Some viral infections may produce benign infantile seizures.

Paratyphoid fever resembles typhoid fever. Infection is characterized by a sustained fever, headache, abdominal pain, malaise, anorexia, a nonproductive cough (in early stage of illness), a relative bradycardia (slow heart rate), and hepatosplenomegaly (an enlargement of the liver and spleen). About 30% of Caucasians develop rosy spots on the central body. In adults, constipation is more common than diarrhea.

Only 20% to 40% of people initially have abdominal pain. Nonspecific symptoms such as chills, sweating, headache, loss of appetite, cough, weakness, sore throat, dizziness, and muscle pains are frequently present before the onset of fever. Some very rare symptoms are psychosis (mental disorder), confusion, and seizures.

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

Intestinal infectious diseases include a large number of infections of the bowels including: cholera, typhoid fever, paratyphoid fever, other types of salmonella infections, shigellosis, botulism, gastroenteritis, and amoebiasis among others.

Typhoid and paratyphoid resulted in 221,000 deaths in 2013 down from 259,000 deaths in 1990. Other diseases which result in diarrhea caused another 1.3 million additional deaths in 2013 down from 2.6 million deaths in 1990.

Paratyphoid fever, also known simply as paratyphoid, is a bacterial infection caused by one of the three types of "Salmonella enterica". Symptoms usually begin six to thirty days after exposure and are the same as those of typhoid fever. Often there is a gradual onset of a high fever over several days. Weakness, loss of appetite, and headaches also commonly occur. Some people develop a skin rash with rose colored spots. Without treatment symptoms may last weeks or months. Other people may carry the bacteria without being affected; however, are still able to spread the disease to others. Both typhoid and paratyphoid are of similar severity. Paratyphoid fever is a type of enteric fever along with typhoid fever.

Paratyphoid is caused by the bacteria "Salmonella enterica" of the serotype Paratyphi A, Paratyphi B or Paratyphi C growing in the intestines and blood. They are usually spread by eating or drinking food or water contaminated with the feces of an infected person. They may occur when a person who prepares food is infected. Risk factors include poor sanitation as is found among poor crowded populations. Occasionally they may be transmitted by sex. Humans are the only animal infected. Diagnosis may be based on symptoms and confirmed by either culturing the bacteria or detecting the bacteria's DNA in the blood, stool, or bone marrow. Culturing the bacteria can be difficult. Bone marrow testing is the most accurate. Symptoms are similar to that of many other infectious diseases. Typhus is an unrelated disease.

While there is no vaccine specifically for paratyphoid, the typhoid vaccine may provide some benefit. Prevention includes drinking clean water, better sanitation, and better handwashing. Treatment of the disease is with antibiotics such as azithromycin. Resistance to a number of other previously effective antibiotics is common.

Paratyphoid affects about 6 million people a year. It is most common in parts of Asia and rare in the developed world. Most cases are due to Paratyphi A rather than Paratyphi B or C. In 2015 paratyphoid fever resulted in about 29,200 deaths down from 63,000 deaths in 1990. The risk of death is between 10% and 15% without treatment while with treatment it may be less than one percent.

Patients can develop two clinical phases: an acute septic phase and a chronic eruptive phase associated with skin lesions. In the acute phase (also known as Oroya fever or "fiebre de la Oroya"), "B. bacilliformis" infection is a sudden, potentially life-threatening infection associated with high fever and decreased levels of circulating red blood cells (i.e., hemolytic anemia)and transient immunosuppression. "B. bacilliformis" is considered the most deadly species to date, with a death rate of up to 90% during the acute phase, which typically lasts two to four weeks. Peripheral blood smears show anisomacrocytosis with many bacilli adherent to red blood cells. Thrombocytopenia is also seen and can be very severe. Neurologic manifestations (neurobartonellosis) are altered mental status, agitation, or even coma, ataxia, spinal meningitis, or paralysis. It is seen in 20% of patients with acute infection, in which the prognosis is very guarded with an about 50% mortality. The most feared complication is overwhelming infection mainly by Enterobacteriaceae, particularly "Salmonella" (both "S. typhi" and " S. "non-"typhi", as well as reactivation of toxoplasmosis and other opportunistic infections .

The chronic manifestation consists of a benign skin eruption with raised, reddish-purple nodules (angiomatous tumours). The bacterium can be seen microscopically, if a skin biopsy is silver stained (the Warthin–Starry method).

Signs and symptoms of enteritis are highly variable and vary based on the specific cause and other factors such as individual variance and stage of disease.

Symptoms may include abdominal pain, cramping, diarrhoea, dehydration, fever, nausea, vomiting and weight loss.

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".