The portal of entry is the gastrointestinal tract. The organism is acquired usually by insufficiently cooked pork or contaminated water, meat, or milk. Acute "Y. enterocolitica" infections usually lead to mild self-limiting enterocolitis or terminal ileitis and adenitis in humans. Symptoms may include watery or bloody diarrhea and fever, resembling appendicitis or salmonellosis or shigellosis. After oral uptake, "Yersinia" species replicate in the terminal ileum and invade Peyer's patches. From here they can disseminate further to mesenteric lymph nodes causing lymphadenopathy. This condition can be confused with appendicitis, so is called pseudoappendicitis. In immunosuppressed individuals, they can disseminate from the gut to the liver and spleen and form abscesses. Because "Yersinia" species are siderophilic (iron-loving) bacteria, people with hereditary hemochromatosis (a disease resulting in high body iron levels) are more susceptible to infection with "Yersinia" (and other siderophilic bacteria). In fact, the most common contaminant of stored blood is "Y. enterocolitica". See yersiniosis for further details.

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.

Yersinia enterocolitica is a Gram-negative bacillus-shaped bacterium, belonging to the family Enterobacteriaceae. It is motile at temperatures of 22–29°C, but becomes nonmotile at normal human body temperature"." "Y. enterocolitica" infection causes the disease yersiniosis, which is an animal-borne disease occurring in humans, as well as in a wide array of animals such as cattle, deer, pigs, and birds. Many of these animals recover from the disease and become carriers; these are potential sources of contagion despite showing no signs of disease. The bacterium infects the host by sticking to its cells using trimeric autotransporter adhesins.

The genus "Yersinia" includes 11 species:

"Y. pestis, Y. pseudotuberculosis, Y. enterocolitica, Y. frederiksenii,"

"Y. intermedia, Y. kristensenii, Y. bercovieri," "Y. mollaretii, Y. rohdei, Y. aldovae", and "Y. ruckeri". Among them, only "Y. pestis, Y. pseudotuberculosis", and certain strains of "Y. enterocolitica" are of pathogenic importance for humans and certain warm-blooded animals, whereas the other species are of environmental origin and may, at best, act as opportunists. However, "Yersinia" strains can be isolated from clinical materials, so have to be identified at the species level.

"Y. enterocolitica" is a heterogeneous group of strains, which are traditionally classified by biotyping into six biogroups on the basis of phenotypic characteristics, and by serotyping into more than 57 O serogroups, on the basis of their O (lipopolysaccharide or LPS) surface antigen. Five of the six biogroups (1B and 2–5) are regarded as pathogens. However, only a few of these serogroups have been associated with disease in either humans or animals. Strains that belong to serogroups O:3 (biogroup 4), O:5,27 (biogroups 2 and 3), O:8 (biogroup 1B), and O:9 (biogroup 2) are most frequently isolated worldwide from human samples. However, the most important "Y. enterocolitica" serogroup in many European countries is serogroup O:3 followed by O:9, whereas the serogroup O:8 is mainly detected in the United States.

"Y. enterocolitica" is widespread in nature, occurring in reservoirs ranging from the intestinal tracts of numerous mammals, avian species, cold-blooded species, and even from terrestrial and aquatic niches. Most environmental isolates are avirulent; however, isolates recovered from porcine sources contain human pathogenic serogroups. In addition, dogs, sheep, wild rodents, and environmental water may also be a reservoir of pathogenic "Y. enterocolitica "strains. Human pathogenic

strains are usually confined to the intestinal tract and lead to enteritis/diarrhea.

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.

Yersinia pseudotuberculosis is a Gram-negative bacterium that causes Far East scarlet-like fever in humans, who occasionally get infected zoonotically, most often through the food-borne route. Animals are also infected by "Y. pseudotuberculosis". The bacterium is urease positive.

In animals, "Y. pseudotuberculosis" can cause tuberculosis-like symptoms, including localized tissue necrosis and granulomas in the spleen, liver, and lymph nodes.

In humans, symptoms of Far East scarlet-like fever are similar to those of infection with "Yersinia enterocolitica" (fever and right-sided abdominal pain), except that the diarrheal component is often absent, which sometimes makes the resulting condition difficult to diagnose. "Y. pseudotuberculosis" infections can mimic appendicitis, especially in children and younger adults, and, in rare cases, the disease may cause skin complaints (erythema nodosum), joint stiffness and pain (reactive arthritis), or spread of bacteria to the blood (bacteremia).

Far East scarlet-like fever usually becomes apparent five to 10 days after exposure and typically lasts one to three weeks without treatment. In complex cases or those involving immunocompromised patients, antibiotics may be necessary for resolution; ampicillin, aminoglycosides, tetracycline, chloramphenicol, or a cephalosporin may all be effective.

The recently described syndrome "Izumi-fever" has been linked to infection with "Y. pseudotuberculosis".

The symptoms of fever and abdominal pain mimicking appendicitis (actually from mesenteric lymphadenitis) associated with "Y. pseudotuberculosis" infection are not typical of the diarrhea and vomiting from classical food poisoning incidents. Although "Y. pseudotuberculosis" is usually only able to colonize hosts by peripheral routes and cause serious disease in immunocompromised individuals, if this bacterium gains access to the blood stream, it has an LD comparable to "Y. pestis" at only 10 CFU.

Infection can cause subcutaneous haemorrhage that presents as reddening of the throat, mouth, gill tips, and fins, and eventual erosion of the jaw and palate. Hemorrhaging also occurs on internal organs, and in the later stages of the disease, the abdomen becomes filled with a yellow fluid - giving the fish a "pot-bellied" appearance. The fish often demonstrate abnormal behavior and anorexia. Mortality rates can be high.

A presumptive diagnosis can be made based in the history and clinical signs, but definitive diagnosis requires bacterial culture and serological testing such as ELISA and latex agglutination.

Some fish species serve as vectors for the disease and have subsequently spread the pathogen to other parts of the world. An example is the fathead minnow ("Pimephales promelas") which is responsible for the spread of redmouth disease to trout in Europe. Other vectors include the goldfish ("Carassius auratus"), Atlantic and Pacific salmon ("Salmo salar"), the emerald shiner ("Notropis atherinoides"), and farmed whitefish ("Coregonus" spp.). Infections have also occurred in farmed turbot ("Scophthalmus maximus"), seabass ("Dicentrarchus labrax"), and seabream ("Sparus auratus"). It can now be found in North and South America, Africa, Asia, and Australia, as well as Europe.

The symptoms are like those associated with many other febrile diseases, but with emphasis on muscular pain and night sweats. The duration of the disease can vary from a few weeks to many months or even years.

In the first stage of the disease, sepsis occurs and leads to the classic triad of undulant fevers, sweating (often with characteristic foul moldy smell sometimes likened to wet hay), and migratory arthralgia and myalgia (joint and muscle pain). Blood tests characteristically reveal a low number of white blood cells and red blood cells, show some elevation of liver enzymes such as aspartate aminotransferase (AST) and alanine aminotransferase (ALT), and demonstrate positive Bengal Rose and Huddleston reactions. Gastrointestinal symptoms occur in 70% of cases and include nausea, vomiting, decreased appetite, unintentional weight loss, abdominal pain, constipation, diarrhea, an enlarged liver, liver inflammation, liver abscess, and an enlarged spleen.

This complex is, at least in Portugal, Israel, and Jordan, known as Malta fever. During episodes of Malta fever, melitococcemia (presence of brucellae in blood) can usually be demonstrated by means of blood culture in tryptose medium or Albini medium. If untreated, the disease can give origin to focalizations or become chronic. The focalizations of brucellosis occur usually in bones and joints and spondylodiscitis of the lumbar spine accompanied by sacroiliitis is very characteristic of this disease. Orchitis is also common in men.

Diagnosis of brucellosis relies on:

1. Demonstration of the agent: blood cultures in tryptose broth, bone marrow cultures. The growth of brucellae is extremely slow (they can take up to two months to grow) and the culture poses a risk to laboratory personnel due to high infectivity of brucellae.

2. Demonstration of antibodies against the agent either with the classic Huddleson, Wright, and/or Bengal Rose reactions, either with ELISA or the 2-mercaptoethanol assay for IgM antibodies associated with chronic disease

3. Histologic evidence of granulomatous hepatitis on hepatic biopsy

4. Radiologic alterations in infected vertebrae: the Pedro Pons sign (preferential erosion of the anterosuperior corner of lumbar vertebrae) and marked osteophytosis are suspicious of brucellic spondylitis.

The consequences of "Brucella" infection are highly variable and may include arthritis, spondylitis, thrombocytopenia, meningitis, uveitis, optic neuritis, endocarditis, and various neurological disorders collectively known as neurobrucellosis.

The usual symptoms are:

- Abdominal pain

- Bleeding under skin due to blood clotting problems

- Bleeding from mouth, nose or rectum

- Diarrhea

- Fever

- Chills

- Low blood pressure

- Nausea

- Organ failure

- Vomiting

- Shock

- Death of tissue (gangrene) causing blackening in extremities, mostly fingers, toes and nose

- Difficulty breathing

However, septicemic plague may cause death before any symptoms occur. Also, the above symptoms are common to many human illnesses, and are not considered diagnostic of any form of plague.

These include

- red skin rash usually of the face, elbows, and knees

- skin desquamation

- exanthema

- red tongue

- toxic shock syndrome

Other features include mesenteric lymphadenitis and arthritis. Kidney failure rarely occurs.

Relapses occur in up to 50% of patients.

Brucellosis is a highly contagious zoonosis caused by ingestion of unpasteurized milk or undercooked meat from infected animals, or close contact with their secretions.

"Brucella" species are small, gram-negative, nonmotile, nonspore-forming, rod-shaped (coccobacilli) bacteria. They function as facultative intracellular parasites, causing chronic disease, which usually persists for life. Four species infect humans: "B. abortus", "B. canis", "B. melitensis", and "B. suis". "B. abortus" is less virulent than "B. melitensis" and is primarily a disease of cattle. "B. canis" affects dogs. "B. melitensis" is the most virulent and invasive species; it usually infects goats and occasionally sheep. "B. suis" is of intermediate virulence and chiefly infects pigs. Symptoms include profuse sweating and joint and muscle pain. Brucellosis has been recognized in animals and humans since the 20th century.

Sylvatic plague is an infectious bacterial disease caused by the bacterium "Yersinia pestis" that primarily affects rodents such as prairie dogs. It is the same bacterium that causes bubonic and pneumonic plague in humans. Sylvatic, or sylvan, means 'occurring in wildlife,' and refers specifically to the form of plague in rural wildlife. Urban plague refers to the form in urban wildlife.

It is primarily transmitted among wildlife through flea bites and contact with infected tissue or fluids. Sylvatic plague is most commonly found in prairie dog colonies and some mustelids like the black-footed ferret.

Septicemic plague is one of the three main forms of plague. It is caused by "Yersinia pestis", a gram-negative species of bacterium. Septicemic plague is a life-threatening infection of the blood, most commonly spread by bites from infected fleas.

Like some other forms of gram-negative sepsis, septicemic plague can cause disseminated intravascular coagulation, and is almost always fatal when untreated (the mortality rate in medieval times was 99–100 percent). However, it only occurs in a minority of cases of "Yersinia" infection, so that fewer than 5,000 people a year acquire the disease. It is the rarest of the three plague varieties; the other forms are bubonic and pneumonic plague.

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.

Urban plague is an infectious disease among rodent species that live in close association with humans in urban areas. It is caused by the bacterium Yersinia pestis which is the same bacterium that causes bubonic and pneumonic plague in humans. Plague was first introduced into the United States in 1900 by rat–infested steamships that had sailed from affected areas, mostly from Asia. Urban plague spread from urban rats to rural rodent species, especially among prairie dogs in the western United States.

Common vectors for urban plague are house mice, black rats, and Norway rats.

"Streptococcus pneumoniae" () is the most common bacterial cause of pneumonia in all age groups except newborn infants. "Streptococcus pneumoniae" is a Gram-positive bacterium that often lives in the throat of people who do not have pneumonia.

Other important Gram-positive causes of pneumonia are "Staphylococcus aureus" () and "Bacillus anthracis".

An emerging infectious disease (EID) is an infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human pathogens. EIDs are caused by newly identified species or strains (e.g. Severe acute respiratory syndrome, HIV/AIDS) that may have evolved from a known infection (e.g. influenza) or spread to a new population (e.g. West Nile fever) or to an area undergoing ecologic transformation (e.g. Lyme disease), or be "reemerging" infections, like drug resistant tuberculosis. Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and non-infectious conditions leading to the development of novel syndemics. Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.

Bacterial pneumonia is a type of pneumonia caused by bacterial infection.

Sylvatic plague is most commonly found in prairie dog colonies; the flea that feeds on prairie dogs (and other mammals) serves as the vector for transmission to the new host.

The most apparent symptom of pneumonic plague is coughing, often with hemoptysis (coughing up blood). With pneumonic plague, the first signs of illness are fever, headache, weakness and rapidly developing pneumonia with shortness of breath, chest pain, cough and sometimes bloody or watery sputum.

The pneumonia progresses for two to four days and may cause respiratory failure and shock. Patients will die without early treatment, some within 36 hours.

Initial pneumonic plague symptoms can often include the following:

- Fever

- Weakness

- Headaches

- Nausea

Rapidly developing pneumonia with:

- Shortness of breath

- Chest pain

- Cough

- Bloody or watery sputum (saliva and discharge from respiratory passages).

The average incubation periods for giardiasis and cryptosporidiosis are each 7 days. Certain other bacterial and viral agents have shorter incubation periods, although hepatitis may take weeks to manifest itself. The onset usually occurs within the first week of return from the field, but may also occur at any time while hiking.

Most cases begin abruptly and usually result in increased frequency, volume, and weight of stool. Typically, a hiker experiences at least four to five loose or watery bowel movements each day. Other commonly associated symptoms are nausea, vomiting, abdominal cramping, bloating, low fever, urgency, and malaise, and usually the appetite is affected. The condition is much more serious if there is blood or mucus in stools, abdominal pain, or high fever. Dehydration is a possibility. Life-threatening illness resulting from WAD is extremely rare but can occur in people with weakened immune systems.

Some people may be carriers and not exhibit symptoms.

Lymphatics ultimately drain into the bloodstream, so the plague bacteria may enter the blood and travel to almost any part of the body. In septicemic plague, bacterial endotoxins cause disseminated intravascular coagulation (DIC), causing tiny clots throughout the body and possibly ischaemic necrosis (tissue death due to lack of circulation/perfusion to that tissue) from the clots. DIC results in depletion of the body's clotting resources, so that it can no longer control bleeding. Consequently, there is bleeding into the skin and other organs, which can cause red and/or black patchy rash and hemoptysis/hematemesis (coughing up/ vomiting of blood). There are bumps on the skin that look somewhat like insect bites; these are usually red, and sometimes white in the center. Untreated, septicemic plague is usually fatal. Early treatment with antibiotics reduces the mortality rate to between 4 and 15 percent. People who die from this form of plague often die on the same day symptoms first appear.

The pneumonic form of plague arises from infection of the lungs. It causes coughing and sneezing and thereby produces airborne droplets that contain bacterial cells and are likely to infect anyone inhaling them. The incubation period for pneumonic plague is short, usually two to four days, but sometimes just a few hours. The initial signs are indistinguishable from several other respiratory illnesses; they include headache, weakness and spitting or vomiting of blood. The course of the disease is rapid; unless diagnosed and treated soon enough, typically within a few hours, death may follow in one to six days; in untreated cases mortality is nearly 100%.