The disease primarily affects older adults, persons with weakened immune systems, pregnant women, and newborns. Rarely, people without these risk factors can also be affected. A person with listeriosis usually has fever and muscle aches, often preceded by diarrhea or other gastrointestinal symptoms. Almost everyone who is diagnosed with listeriosis has invasive infection (meaning that the bacteria spread from their intestines to their blood stream or other body sites). Disease may occur as much as two months after eating contaminated food.

The symptoms vary with the infected person:

- High-risk persons other than pregnant women: Symptoms can include fever, muscle aches, headache, stiff neck, confusion, loss of balance, and convulsions.

- Pregnant women: Pregnant women typically experience only a mild, flu-like illness. However, infections during pregnancy can lead to miscarriage, stillbirth, premature delivery, or life-threatening infection of the newborn.

- Previously healthy persons: People who were previously healthy but were exposed to a very large dose of Listeria can develop a non-invasive illness (meaning that the bacteria have not spread into their blood stream or other body sites). Symptoms can include diarrhea and fever.

If an animal has eaten food contaminated with Listeria and does not have any symptoms, most experts believe that no tests or treatment are needed, even for people at high risk for listeriosis.

In sheep, the disease is also called the "circling disease". The most obvious signs for the veterinarians are neurological, especially lateral deviation of the neck and head.

Listeriosis is an infectious but not contagious disease caused by the bacterium "Listeria monocytogenes", far more common in domestics animals (domestic mammals and poultry), especially ruminants, than in human beings. It can also occur in feral animals—among others, game animals—as well as in poultry and other birds.

The causative bacterium lives in the soil and in poorly made silage, and is acquired by ingestion. It is not contagious; over the course of a 30-year observation period of sheep disease in Morocco, the disease only appeared in the late 2000s (decade) when feeding bag-ensiled corn became common. In Iceland, the disease is called "silage sickness".

The disease is sporadic, but can occur as farm outbreaks in ruminants.

Three main forms are usually recognized throughout the affected species:

- encephalitis, the most common form in ruminants

- late abortion

- gastro-intestinal septicemia with liver damage, in monogastric species as well as in preruminant calves and lambs

Listeriosis in animals can sometimes be cured with antibiotics (tetracyclines, chloramphenicol and benzyl penicillin) when diagnosed early. Goats, for example, can be treated upon first noticing the disease's characteristic expression in the animal's face, but is generally fatal.

Listeriosis is a bacterial infection most commonly caused by "Listeria monocytogenes", although "L. ivanovii" and "L. grayi" have been reported in certain cases. "Listeria" primarily causes infections of the central nervous system (meningitis, meningoencephalitis, brain abscess, cerebritis) and bacteremia in those who are immunocompromised, pregnant women, and those at the extremes of age (newborns and the elderly), as well as gastroenteritis in healthy persons who have been severely infected. "Listeria" is ubiquitous and is primarily transmitted via the oral route after ingestion of contaminated food products, after which the organism penetrates the intestinal tract to cause systemic infections. The diagnosis of listeriosis requires the isolation of the organism from the blood and/or the cerebrospinal fluid. Treatment includes prolonged administration of antibiotics, primarily ampicillin and gentamicin, to which the organism is usually susceptible.

Tyzzer’s disease is an acute epizootic bacterial disease found in rodents, rabbits, dogs, cats, birds, pandas, deer, foals, cattle, and other mammals including gerbils. It is caused by the spore-forming bacterium "Clostridium piliforme", formerly known as "Bacillus piliformis". It is an infectious disease characterized by necrotic lesions on the liver, is usually fatal, and is present worldwide. Animals with the disease become infected through oral ingestion of the bacterial spores and usually die within a matter of days. Animals most commonly affected include young, stressed animals in laboratory environments, such as immature rodents and rabbits. Most commonly affected wild animals include muskrats "(Ondatra zibethicus)" and occasionally cottontail rabbits "(Lepus sylvaticus)". Even today, much remains unknown about Tyzzer’s disease, including how and why it occurs.

Common clinical signs of Tyzzer’s Disease include watery diarrhea, depression, emaciation, and a ruffled coat. Other observed clinical signs include melena, depression, lethargy, and decreased temperature. In muskrats, this disease is characterized by extensive hemorrhaging within the lower intestine and abdomen. Due to the fast-acting nature of this disease, infected individuals often do not live long enough to exhibit symptoms. It is not uncommon for an infected animal to die within 1-10 days of disease contraction.

During necropsy, inflammation of the ileum, cecum, and colon are commonly present. Perhaps the most distinctive trait of this disease, however, is the grayish yellow necrotic lesions found on the liver of diseased animals. The number of these spots present can range from one to countless. Occasionally, lesions are discovered in the lower intestinal tract and heart as well. Even with physical signs and symptoms present, a conclusive diagnosis is dependent upon the presence of "C. piliforme" within the liver of the infected animal.

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.

After an incubation period of up to seven days, the signs associated with swine vesicular disease occur. The first sign is a transient mild fever. Other signs include:

- Vesicles in the mouth and on the snout and feet

- Lameness and an unsteady gait, shivering and jerking–type leg movements

- Ruptured vesicles can cause ulcers on limbs and feet, and foot pads may be loosened.

Young animals are more severely affected. Recovery often occurs within a week. There is no mortality.

Swine vesicular disease has the same clinical signs as foot-and-mouth disease, and can only be diagnosed by laboratory testing.

In cattle, the main signs of paratuberculosis are diarrhea and wasting. Most cases are seen in 2- to 6-year-old animals. The initial signs can be subtle, and may be limited to weight loss, decreased milk production, or roughening of the hair coat. The diarrhea is usually thick, without blood, mucus, or epithelial debris, and may be intermittent. Several weeks after the onset of diarrhea, a soft swelling may occur under the jaw. Known as "bottle jaw" or intermandibular edema, this symptom is due to protein loss from the bloodstream into the digestive tract. Paratuberculosis is progressive; affected animals become increasingly emaciated and usually die as the result of dehydration and severe cachexia.

Signs are rarely evident until two or more years after the initial infection, which usually occurs shortly after birth. Animals are most susceptible to the infection in the first year of life. Newborns most often become infected by swallowing small amounts of infected manure from the birthing environment or udder of the mother. In addition, newborns may become infected while in the uterus or by swallowing bacteria passed in milk and colostrum. Animals exposed at an older age, or exposed to a very small dose of bacteria at a young age, are not likely to develop clinical disease until they are much older than two years.

The clinical signs are similar in other ruminants. In sheep and goats, the wool or hair is often damaged and easily shed, and diarrhea is uncommon. In deer, paratuberculosis can progress rapidly. Intestinal disease has also been reported in rabbits and nonhuman primates.

Unlike cattle and sheep, infections in deer often present with clinical illness in animals under one year of age.

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

Paratuberculosis or Johne's disease is a contagious, chronic and sometimes fatal infection that primarily affects the small intestine of ruminants. It is caused by the bacterium "Mycobacterium avium" subspecies "paratuberculosis". Infections normally affect ruminants (mammals that have four compartments of their stomachs, of which the rumen is one), but have also been seen in a variety of nonruminant species, including rabbits, foxes, and birds. Horses, dogs, and nonhuman primates have been infected experimentally. Paratuberculosis is found worldwide, with some states in Australia (where it is usually called bovine Johne's disease or BJD) as the only areas proven to be free of the disease.

Some sources define "paratuberculosis" by the lack of "Mycobacterium tuberculosis", rather than the presence of any specific infectious agent, leaving ambiguous the appropriateness of the term to describe Buruli ulcer or Lady Windermere syndrome.

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.

In sheep, BTV causes an acute disease with high morbidity and mortality. BTV also infects goats, cattle and other domestic animals as well as wild ruminants (for example, blesbuck, white-tailed deer, elk, and pronghorn antelope).

Major signs are high fever, excessive salivation, swelling of the face and tongue and cyanosis of the tongue. Swelling of the lips and tongue gives the tongue its typical blue appearance, though this sign is confined to a minority of the animals. Nasal signs may be prominent, with nasal discharge and stertorous respiration.

Some animals also develop foot lesions, beginning with coronitis, with consequent lameness. In sheep, this can lead to knee-walking. In cattle, constant changing of position of the feet gives bluetongue the nickname The Dancing Disease. Torsion of the neck (opisthotonos or torticollis) is observed in severely affected animals.

Not all animals develop signs, but all those that do lose condition rapidly, and the sickest die within a week. For affected animals which do not die, recovery is very slow, lasting several months.

The incubation period is 5–20 days, and all signs usually develop within a month. The mortality rate is normally low, but it is high in susceptible breeds of sheep. In Africa, local breeds of sheep may have no mortality, but in imported breeds it may be up to 90 percent.

In cattle, goats and wild ruminants infection is usually asymptomatic despite high virus levels in blood. Red deer are an exception, and in them the disease may be as acute as in sheep.

Swine vesicular disease (SVD) is an acute, contagious viral disease of swine caused by the swine vesicular disease virus, an enterovirus. It is characterized by fever and vesicles with subsequent ulcers in the mouth and on the snout, feet, and teats. The pathogen is relatively resistant to heat, and can persist for a long time in salted, dried, and smoked meat products. Swine vesicular disease does not cause economically-important disease, but is important due to its similarity to foot-and-mouth disease.

On post-mortem examination (necropsy), the most obvious gross lesion is subcutaneous oedema in the submandibular and pectoral (brisket) regions. Petechial haemorrhages are found subcutaneously and in the thoracic cavity. In addition, congestion and various degrees of consolidation of the lung may occur. Animals that die within 24–36 hours, have only few petechial haemorrhages on the heart and generalised congestion of the lung, while in animals that die after 72 hours, petechial and ecchymotic haemorrhages were more evident and lung consolidation are more extensive.

Affected animals normally have generalised signs such as depression, dullness, weakness and lethargy, pyrexia and weight loss and decreased production. They will also have respiratory signs including bilateral nasal discharge, dyspnoea, tachypnoea and coughing. Occasionally the only sign seen is sudden death.

Typical pathological lesions are very suggestive of the disease - they are localised exclusively to the lung and pleura. Lungs are normally a port wine colour and abundant pleural exudate and pleuritis and adhesions are common. The pleural exudates may have solidified forming a gelatinous covering.

Histological examination of the lung tissues may show acute serofibrinous to chronic fibrino-necrotic pleuropneumonia with neutrophilic inflammation in the alveoli, bronchioles, interstitial septae and subpleural connective tissue.

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.

An anthroponotic disease, or anthroponosis, is an infectious disease in which a disease causing agent carried by humans is transferred to other animals. It may cause the same disease or a different disease in other animals. Since humans do not generally inflict bite wounds on other animals, the method of transmissions is always a "soft" contact such as skin to skin transmission. An example is chytridiomycosis which can be spread by humans with the fungus on their skin handling frogs with bare hands.

The reverse situation, a disease transmitted from animals to humans, is known as zoonotic.

It can also be defined as a human-to-human infection with no animal vector.

Exotic ungulate encephalopathy is a transmissible spongiform encephalopathy (TSE), or prion disease, identified in infected organs of zoo animals. This subgroup of the TSEs in captive animals was identified in zoo animals in Great Britain including species of greater kudu, nyala, gemsbok, the common eland, Arabian and Scimitar Oryx, an Ankole-Watusi cow, and an American bison. Studies indicate that transmission likely occurred via the consumption of feed supplemented with meat and bone meal, although some animals died after the British ban on ground offal in animal feed. All animals died during the 1990s, with the last death occurring in 1998.

Zoonotic transmission can occur in any context in which there is companionistic (pets), economic (farming, etc.), predatory (hunting, butchering or consuming wild game) or research contact with or consumption of animals, animal products, or animal derivatives (vaccines, etc.).

Surra (from the Marathi "sūra", meaning the sound of heavy breathing through nostrils, of imitative origin) is a disease of vertebrate animals. The disease is caused by protozoan trypanosomes, specifically "Trypanosoma evansi", of several species which infect the blood of the vertebrate host, causing fever, weakness, and lethargy which lead to weight loss and anemia. In some animals the disease is fatal unless treated.

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.

A wide variety of clinical signs have been described for HS in cattle and buffaloes. The incubation periods (the time between exposure and observable disease) for buffalo calves 4–10 months of age varies according to the route of infection. The incubation period is 12–14 hours, approximately 30 hours and 46–80 hours for subcutaneous infection, oral infection and natural exposure, respectively.

There is variability in the duration of the clinical course of the disease. In the case of experimental subcutaneous infection, the clinical course lasted only a few hours, while it persisted for 2–5 days following oral infection and in buffaloes and cattle that had been exposed to naturally-infected animals. It has also been recorded from field observations that the clinical courses of per-acute and acute cases were 4–12 hours and 2–3 days, respectively.

Generally, progression of the disease in buffaloes and cattle is divided into three phases. Phase one is characterised by fever, with a rectal temperature of , loss of appetite and depression. Phase two is typified by increased respiration rate (40–50/minute), laboured breathing, clear nasal discharge (turns opaque and mucopurulent as the disease progresses), salivation and submandibular oedema spreading to the pectoral (brisket) region and even to the forelegs. Finally, in phase three, there is typically recumbency, continued acute respiratory distress and terminal septicaemia. The three phases overlap when the disease course is short. In general, buffaloes have a more acute onset of disease than cattle, with a shorter duration.

Zoonoses are infectious diseases of animals (usually vertebrates) that can naturally be transmitted to humans.

Major modern diseases such as Ebola virus disease and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved to a separate human-only disease. Most strains of influenza that infect humans are human diseases, although many strains of swine and bird flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause pandemics such as the 1918 Spanish flu or the 2009 swine flu. "Taenia solium" infection is one of the neglected tropical diseases with public health and veterinary concern in endemic regions. Zoonoses can be caused by a range of disease pathogens such as viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in animals; however, only diseases that routinely involve animal to human transmission, like rabies, are considered direct zoonosis.

Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting infected. When humans infect animals, it is called reverse zoonosis or anthroponosis. The term is from Greek: ζῷον "zoon" "animal" and νόσος "nosos" "sickness".

Cat bites are usually considered as minor injuries but can result in serious infection. Not all infections that can be obtained from exposure to a cat are transmitted through a cat bite, like plague.