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.

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.

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).

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 best-known symptom of bubonic plague is one or more infected, enlarged, and painful lymph nodes, known as buboes. After being transmitted via the bite of an infected flea, the "Y. pestis" bacteria become localized in an inflamed lymph node where they begin to colonize and reproduce. Buboes associated with the bubonic plague are commonly found in the armpits, upper femoral, groin and neck region. Acral gangrene (i.e., of the fingers, toes, lips and nose) is another common symptom.

Because of its bite-based mode of transmission, the bubonic plague is often the first of a progressive series of illnesses. Bubonic plague symptoms appear suddenly a few days after exposure to the bacterium. Symptoms include:

- Chills

- General ill feeling (malaise)

- High fever (39 °C; 102 °F)

- Muscle cramps

- Seizures

- Smooth, painful lymph gland swelling called a bubo, commonly found in the groin, but may occur in the armpits or neck, most often near the site of the initial infection (bite or scratch)

- Pain may occur in the area before the swelling appears

- Gangrene of the extremities such as toes, fingers, lips and tip of the nose.

Other symptoms include heavy breathing, continuous vomiting of blood (hematemesis), aching limbs, coughing, and extreme pain caused by the decay or decomposition of the skin while the person is still alive. Additional symptoms include extreme fatigue, gastrointestinal problems, lenticulae (black dots scattered throughout the body), delirium, and coma.

Depending on the site of infection, tularemia has six characteristic clinical variants: ulceroglandular (the most common type representing 75% of all forms), glandular, oropharyngeal, pneumonic, oculoglandular, and typhoidal.

The incubation period for tularemia is one to 14 days; most human infections become apparent after three to five days. In most susceptible mammals, the clinical signs include fever, lethargy, loss of appetite, signs of sepsis, and possibly death. Nonhuman mammals rarely develop the skin lesions seen in people. Subclinical infections are common, and animals often develop specific antibodies to the organism. Fever is moderate or very high, and tularemia bacilli can be isolated from blood cultures at this stage. The face and eyes redden and become inflamed. Inflammation spreads to the lymph nodes, which enlarge and may (mimicking bubonic plague). Lymph node involvement is accompanied by a high fever.

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%.

Respiratory infection in humans is relatively rare and presents as two stages. It infects the lymph nodes in the chest first, rather than the lungs themselves, a condition called hemorrhagic mediastinitis, causing bloody fluid to accumulate in the chest cavity, therefore causing shortness of breath. The first stage causes cold and flu-like symptoms. Symptoms include fever, shortness of breath, cough, fatigue, and chills. This can last hours to days. Often, many fatalities from inhalational anthrax are when the first stage is mistaken for the cold or flu and the victim does not seek treatment until the second stage, which is 90% fatal. The second (pneumonia) stage occurs when the infection spreads from the lymph nodes to the lungs. Symptoms of the second stage develop suddenly after hours or days of the first stage. Symptoms include high fever, extreme shortness of breath, shock, and rapid death within 48 hours in fatal cases. Historical mortality rates were over 85%, but when treated early (seen in the 2001 anthrax attacks), observed case fatality rate dropped to 45%. Distinguishing pulmonary anthrax from more common causes of respiratory illness is essential to avoiding delays in diagnosis and thereby improving outcomes. An algorithm for this purpose has been developed.

In the subgroup of patients where an inoculating event was noted, the mean incubation period of acute melioidosis was 9 days (range 1–21 days). Patients with latent melioidosis may be symptom-free for decades; the longest period between presumed exposure and clinical presentation is 62 years. The potential for prolonged incubation was recognized in US servicemen involved in the Vietnam War, and was referred to as the "Vietnam time-bomb". A wide spectrum of severity exists; in chronic presentations, symptoms may last months, but fulminant infection, particularly associated with near-drowning, may present with severe symptoms over hours.

Symptoms usually appear 2 to 4 weeks after exposure. There are four general types of infection including localized, pulmonary, blood-borne, or disseminated throughout the body. The type and location of the infection usually determines which symptoms appear first as well as which symptoms are more prominent.

Patients with melioidosis usually present with fever. Pain or other symptoms may be suggestive of a clinical focus, which is found in around 75% of patients. Such symptoms include cough or pleuritic chest pain suggestive of pneumonia, bone or joint pain suggestive of osteomyelitis or septic arthritis, or cellulitis. Intra-abdominal infection (including liver and/or splenic abscesses, or prostatic abscesses) do not usually present with focal pain, and imaging of these organs using ultrasound or computed tomography should be performed routinely. In one series of 214 patients, 27.6% had abscesses in the liver or spleen (95% confidence interval, 22.0% to 33.9%). "B. pseudomallei" abscesses may have a characteristic "honeycomb" or "swiss cheese" architecture (hypoechoic, multiseptate, multiloculate) on CT.

Regional variations in disease presentation are seen: parotid abscesses characteristically occur in Thai children, but this presentation has been described only once in Australia. Conversely, prostatic abscesses are found in up to 20% of Australian males, but are rarely described elsewhere. An encephalomyelitis syndrome is recognised in northern Australia.

Patients with melioidosis usually have risk factors for disease, such as diabetes, thalassemia, hazardous alcohol use, or renal disease, and frequently give a history of occupational or recreational exposure to mud or pooled surface water. However, otherwise healthy patients, including children, may also get melioidosis.

In up to 25% of patients, no focus of infection is found and the diagnosis is usually made on blood cultures or throat swab. Melioidosis is said to be able to affect any organ in the body except the heart valves (endocarditis). Although meningitis has been described secondary to ruptured brain abscesses, primary meningitis has not been described. Less common manifestations include intravascular infection, lymph node abscesses (1.2–2.2%), pyopericardium and myocarditis, mediastinal infection, and thyroid and scrotal abscesses and ocular infection.

Cutaneous anthrax, also known as Hide porter's disease, is when anthrax occurs on the skin. It is the most common form (>90% of anthrax cases). Cutaneous anthrax is also the least dangerous form of anthrax (less than 1% mortality rate with treatment). Cutaneous anthrax presents as a boil-like skin lesion that eventually forms an ulcer with a black center (eschar). The black eschar often shows up as a large, painless, necrotic ulcer (beginning as an irritating and itchy skin lesion or blister that is dark and usually concentrated as a black dot, somewhat resembling bread mold) at the site of infection. In general, cutaneous infections form within the site of spore penetration between two and five days after exposure. Unlike bruises or most other lesions, cutaneous anthrax infections normally do not cause pain. Nearby lymph nodes may become infected, reddened, swollen, and painful. A scab forms over the lesion soon, and falls off in a few weeks. Complete recovery may take longer.

Cutaneous anthrax is typically caused when "B. anthracis" spores enter through cuts on the skin. This form is found most commonly when humans handle infected animals and/or animal products.

Cutaneous anthrax is rarely fatal if treated, because the infection area is limited to the skin, preventing the lethal factor, edema factor, and protective antigen from entering and destroying a vital organ. Without treatment, about 20% of cutaneous skin infection cases progress to toxemia and death.

Tularemia, also known as rabbit fever, is an infectious disease caused by the bacterium "Francisella tularensis". Symptoms may include fever, skin ulcer, and enlarged lymph nodes. Occasionally a form that results in pneumonia or a threat infection may occur.

The bacterium is typically spread by ticks, deer flies, or contact with infected animals. It may also be spread by drinking contaminated water or breathing in contaminated dust. It does not spread directly between people. Diagnosis is by blood tests or cultures of the infected site.

Prevention is by using insect repellent, wearing long pants, rapidly removing ticks, and not disturbing dead animals. Treatment is typically with the antibiotic streptomycin. Gentamicin, doxycycline, or ciprofloxacin may also be used.

Between the 1970s and 2015 around 200 cases are reported in the United States a year. Males are affected more often than females. It occurs most frequently in the young and the middle aged. In the United States most cases occur in the summer. The disease is named after Tulare County, California were the disease was discovered in 1911. A number of other animals, such as rabbits, may also be infected.

Pneumonic plague is a severe lung infection caused by the bacterium "Yersinia pestis". Symptoms include fever, headache, shortness of breath, chest pain, and cough. They typically start about three to seven days after exposure. It is one of three forms of plague, the other two being septicemic plague and bubonic plague.

The pneumonic form may occur following an initial bubonic or septicemic plague infection. It may also result from breathing in airborne droplets from another person or cat infected with pneumonic plague. The difference between the forms of plague is the location of infection; in pneumonic plague the infection is in the lungs, in bubonic plague the lymph nodes, and in septicemic plague within the blood. Diagnosis is by testing the blood, sputum, or fluid from a lymph node.

While vaccines are being worked on, in most countries they are not yet commercially available. Prevention is generally by avoiding contact with rodents. It is recommended that those infected be isolated from others. Treatment of pneumonic plague is with antibiotics.

Plague is present among rodents in Africa, the Americas, and Asia. Pneumonic plague is more serious and less common than bubonic plague. The total reported number of all types of plague in 2013 was 783. Untreated pneumonic plague has a mortality of nearly 100%. Some hypothesize that the pneumonic version of the plague was mainly responsible for the Black Death that resulted in approximately 50 million deaths in the 1300s.

Chronic melioidosis is usually defined by a duration of symptoms greater than two months and occurs in about 10% of patients. The clinical presentation of chronic melioidosis is protean and includes such presentations as chronic skin infections, chronic lung nodule, and pneumonia. In particular, chronic melioidosis closely mimics tuberculosis, and has sometimes been called "Vietnamese tuberculosis".

Bubonic plague is one of three types of plague caused by bacterium "Yersinia pestis". One to seven days after exposure to the bacteria, flu like symptoms develop. These include fever, headaches, and vomiting. Swollen and painful lymph nodes occur in the area closest to where the bacteria entered the skin. Occasionally the swollen lymph nodes may break open.

The three types of plague are the result of the route of infection: bubonic plague, septicemic plague, and pneumonic plague. Bubonic plague is mainly spread by infected fleas from small animals. It may also result from exposure to the body fluids from a dead plague infected animal. In the bubonic form of plague, the bacteria enter through the skin through a flea bite and travel via the lymphatic vessels to a lymph node, causing it to swell. Diagnosis is made by finding the bacteria in the blood, sputum, or fluid from lymph nodes.

Prevention is through public health measures such as not handling dead animals in areas where plague is common. Vaccines have not been found to be very useful for plague prevention. Several antibiotics are effective for treatment including streptomycin, gentamicin, and doxycycline. Without treatment it results in the death of 30% to 90% of those infected. Death, if it occurs, is typically within ten days. With treatment the risk of death is around 10%. Globally there are about 650 documented cases a year which result in ~120 deaths. The disease is most common in Africa.

The plague is believed to be the cause of the Black Death that swept through Asia, Europe, and Africa in the 14th century and killed an estimated 50 million people. This was about 25% to 60% of the European population. Because the plague killed so many of the working population, wages rose due to the demand for labor. Some historians see this as a turning point in European economic development. The term "bubonic" is derived from the Greek word , meaning "groin". The term "buboes" is also used to refer to the swollen lymph nodes.

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

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.

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.

The several forms of the infection are:

- Skin/subcutaneous tissue disease is a septic phlegmon that develops classically in the hand and forearm after a cat bite. Inflammatory signs are very rapid to develop; in 1 or 2 hours, edema, severe pain, and serosanguineous exudate appear. Fever, moderate or very high, can be seen, along with vomiting, headache, and diarrhea. Lymphangitis is common. Complications are possible, in the form of septic arthritis, osteitis, or evolution to chronicity.

- Sepsis is very rare, but can be as fulminant as septicaemic plague, with high fever, rigors, and vomiting, followed by shock and coagulopathy.

- Pneumonia disease is also rare and appears in patients with some chronic pulmonary pathology. It usually presents as bilateral consolidating pneumonia, sometimes very severe.

- Zoonosis, pasteurellosis can be transmitted to humans through cats.

Other locations are possible, such as septic arthritis, meningitis, and acute endocarditis, but are very rare.

Leptospiral infection in humans causes a range of symptoms, and some infected persons may have no symptoms at all. Leptospirosis is a biphasic disease that begins suddenly with fever accompanied by chills, intense headache, severe myalgia (muscle ache), abdominal pain, conjunctival suffusion (red eye), and occasionally a skin rash. The symptoms appear after an incubation period of 7–12 days. The first phase (acute or septic phase) ends after 3–7 days of illness. The disappearance of symptoms coincides with the appearance of antibodies against "Leptospira" and the disappearance of all the bacteria from the bloodstream. The patient is asymptomatic for 3–4 days until the second phase begins with another episode of fever. The hallmark of the second phase is meningitis (inflammation of the membranes covering the brain).

Ninety percent of cases of the disease are mild leptospirosis. The rest experience severe disease, which develops during the second stage or occurs as a single progressive illness. The classic form of severe leptospirosis is known as Weil's disease, which is characterized by liver damage (causing jaundice), kidney failure, and bleeding. Additionally, the heart and brain can be affected, meningitis of the outer layer of the brain, encephalitis of brain tissue with same signs and symptoms; and lung affected as the most serious and life-threatening of all leptospirosis complications. The infection is often incorrectly diagnosed due to the nonspecific symptoms.

Other severe manifestations include extreme fatigue, hearing loss, respiratory distress, and azotemia.

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.

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.

Diagnosis is made with isolation of "Pasteurella multocida" in a normally sterile site (blood, pus, or cerebrospinal fluid).

Crayfish plague, "Aphanomyces astaci", is a water mold that infects crayfish, most notably the European "Astacus" which dies within a few weeks of being infected. When experimentally tested, species from Australia, New Guinea and Japan were also found to be susceptible to the infection.

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.

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.