Derzsy's disease is caused by a virus from the Parvoviridae family. It affects geese and Muscovy ducks.

The virus is shed in the faeces and thus transmission is horizontal, via the direct faecal-oral route and also indirectly via fomites. Vertical transmission is also possible.

Clinical disease only occurs in young geese and ducks between birth and 4–5 weeks of age.

Acute disease leads to death in most birds between the ages of 7–10 days. Clinical signs are quite limited in those cases. Older animals tend to show severe systemic and neurological signs and diarrhoea.

Adults do not show any clinical signs.

Viral isolation should be attempted for diagnosis, and immunofluorescence and electron microscopy can confirm the viral infection. Pathological changes may also help the diagnosis.

Clinical appearance of the disease includes depression, a serous nasal discharge, and sporadically minor facial inflammation in mild form of the disease. In severe form, there is severe inflammation of one or both infraorbital sinuses with edema of the surrounding tissue. The swelling can cause closure of one eye or both of them. Intermandibular space and wattles of corks do swell as a course of the disease .

Six syndromes are known to occur after infection with Marek's disease. These syndromes may overlap.

- Classical Marek's disease or neurolymphomatosis causes asymmetric paralysis of one or more limbs. With vagus nerve involvement, difficulty breathing or dilation of the crop may occur. Besides lesions in the peripheral nerves, there are frequently lymphomatous infiltration/tumours in the skin, skeletal muscle, visceral organs. Organs that are commonly affected include the ovary, spleen, liver, kidneys, lungs, heart, proventriculus and adrenals.

- Acute Marek's disease is an epidemic in a previously uninfected or unvaccinated flock, causing depression, paralysis, and death in a large number of birds (up to 80%). The age of onset is much earlier than the classic form; birds are four to eight weeks old when affected. Infiltration into multiple organs/tissue is observed.

- Ocular lymphomatosis causes lymphocyte infiltration of the iris (making the iris turn grey), unequal size of the pupils, and blindness.

- Cutaneous Marek's disease causes round, firm lesions at the feather follicles.

- Atherosclerosis is induced in experimentally infected chickens.

- Immunosuppression – Impairment of the T-lymphocytes prevents competent immunological response against pathogenic challenge and the affected birds become more susceptible to disease conditions such as coccidiosis and "Escherichia coli" infection . Furthermore, without stimulation by cell-mediated immunity, the humoral immunity conferred by the B-cell lines from the Bursa of Fabricius also shuts down, thus resulting in birds that are totally immunocompromised.

Marek's disease is a highly contagious viral neoplastic disease in chickens. It is named after József Marek, a Hungarian veterinarian. Marek's disease is caused by an alphaherpesvirus known as 'Marek's disease virus' (MDV) or "Gallid alphaherpesvirus 2" (GaHV-2). The disease is characterized by the presence of T cell lymphoma as well as infiltration of nerves and organs by lymphocytes. Viruses "related" to MDV appear to be benign and can be used as vaccine strains to prevent Marek's disease. For example, the related Herpesvirus of Turkeys (HVT), causes no apparent disease in turkeys and continues to be used as a vaccine strain for prevention of Marek's disease (see below). Birds infected with GaHV-2 can be carriers and shedders of the virus for life. Newborn chicks are protected by maternal antibodies for a few weeks. After infection, microscopic lesions are present after one to two weeks, and gross lesions are present after three to four weeks. The virus is spread in dander from feather follicles and transmitted by inhalation.

Infectious coryza is a serious bacterial disease of chickens which affects respiratory system and it is manifested by inflammation of the area below the eye, nasal discharge and sneezing...The disease is found all over the world causing high economic losses. Economic loss is due to stumping off and reduction of egg production in case of laying chickens. The disease was discovered early 1930s by considering clinical signs

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.

Diagnosis is based on a circular "bull's-eye" rash at the site of infection called erythema chronicum migrans, which is very similar to that seen in Lyme disease. However, the symptoms of STARI are mild, and resemble influenza, with fatigue, muscle pains, and headache. Fever is sometimes seen, but is not characteristic.

Pogosta disease is a viral disease, established to be identical with other diseases, Karelian fever and Ockelbo disease. The names are derived from the words Pogosta, Karelia and Ockelbo, respectively.

The symptoms of the disease include usually rash, as well as mild fever and other flu-like symptoms; in most cases the symptoms last less than 5 days. However, in some cases, the patients develop a painful arthritis. There are no known chemical agents available to treat the disease.

It has long been suspected that the disease is caused by a Sindbis-like virus, a positive-stranded RNA virus belonging to the Alphavirus genus and family Togaviridae. In 2002 a strain of Sindbis was isolated from patients during an outbreak of the Pogosta disease in Finland, confirming the hypothesis.

This disease is mainly found in the Eastern parts of Finland; a typical Pogosta disease patient is a middle-aged person who has been infected through a mosquito bite while picking berries in the autumn. The prevalence of the disease is about 100 diagnosed cases every year, with larger outbreaks occurring in 7-year intervals.

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.

Pacheco's disease is an acute and often lethal infectious disease in psittacine birds. The disease is caused by a group of herpesviruses, "Psittacid herpesvirus 1" (PsHV-1), which consists of four genotypes. Birds which do not succumb to Pacheco's disease after infection with the virus become asymptomatic carriers that act as reservoirs of the infection. These persistently infected birds, often Macaws, Amazon parrots and some species of conures, shed the virus in feces and in respiratory and oral secretions. Outbreaks can occur when stress causes healthy birds who carry the virus to shed it. Birds generally become infected after ingesting the virus in contaminated material, and show signs of the disease within several weeks.

The main sign of Pacheco's disease is sudden death, sometimes preceded by a short, severe illness. If a bird survives Pacheco's disease following infection with PsHV-1 genotypes 1, 2 or 3, it may later develop internal papilloma disease in the gastrointestinal tract.

Susceptible parrot species include the African gray parrot, and cockatoo. Native Australian birds, such as the eclectus parrot, Bourke's parrot, and budgerigar are susceptible to Pacheco's disease, although the disease itself has not been found in Australia.

Southern tick-associated rash illness (STARI) or Masters' disease is an emerging infectious disease related to Lyme disease that occurs in southeastern and south-central United States. It is spread by tick bites, but the organism that causes the infection is unknown.

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 most common symptoms are diarrhea, abdominal pain, weight loss, and joint pains. The joint pains may be due to migratory non-deforming arthritis, which may occur many years before any digestive tract symptoms develop; they tend to involve the large joints but can occur in any pattern and tend not to damage the joint surface to the point that the joint becomes deformed. Fever and chills occur in a small proportion of people.

In its more advanced form, malabsorption (insufficient absorption of nutrients from the diet) leads to wasting and the enlargement of lymph nodes in the abdomen. Neurological symptoms (discussed below) are more common in those with the severe form of the abdominal disease. Chronic malabsorptive diarrhea leads to the poor absorption of fat, causing steatorrhea (fatty, offensive stool), flatulence, and abdominal distension. Protein-losing enteropathy may also occur, causing depletion of albumin, a blood protein, which may lead to peripheral edema caused by the lowered oncotic pressures.

Hyperpigmentation of the skin occurs in almost half; some also have skin nodules. Various eye problems, such as uveitis, may occur; this is typically associated with deteriorating vision and pain in the affected eye. Endocarditis (infection of the heart valve) has been reported in a small number of cases, sometimes in people with no other symptoms of Whipple's disease; this is typically noticed as breathlessness and leg swelling due to fluid accumulation as the heart is unable to pump fluid through the body.

Of those affected by Whipple's disease, 10–40% of people have problems related to the involvement of the brain; the symptoms relate to the part of the brain that is affected. The most common problems are dementia, memory loss, confusion, and decreased level of consciousness. Eye movement disturbances and myorhythmia (rapidly repetitive movements of the muscles) of the face, together referred to as "oculomasticatory myorhythmia", are highly characteristic for Whipple's disease. Weakness and poor coordination of part of the body, headaches, seizures, as well as a number of more uncommon neurological features, are present in some cases.

Whipple's disease is a rare, systemic infectious disease caused by the bacterium "Tropheryma whipplei". First described by George Hoyt Whipple in 1907 and commonly considered a gastrointestinal disorder, Whipple's disease primarily causes malabsorption but may affect any part of the body including the heart, brain, joints, skin, lungs and the eyes. Weight loss, diarrhea, joint pain, and arthritis are common presenting symptoms, but the presentation can be highly variable and approximately 15% of patients do not have these classic signs and symptoms.

Whipple's disease is significantly more common in men, with 87% of the patients being male. When recognized and treated, Whipple's disease can usually be cured with long-term antibiotic therapy; if the disease is left untreated, it is ultimately fatal.

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.

Feline spongiform encephalopathy is a disease that affects the brains of felines. It is caused by proteins called prions.

Symptoms of meningococcemia are, at least initially, similar to those of influenza. Typically, the first symptoms include fever, nausea, myalgia, headache, arthralgia, chills, diarrhea, stiff neck, and malaise. Later symptoms include septic shock, purpura, hypotension, cyanosis, petechiae, seizures, anxiety, and multiple organ dysfunction syndrome. Acute respiratory distress syndrome and altered mental status may also occur. The petichial rash appear with the 'star-like' shape. Meningococcal sepsis has a greater mortality rate than meningococcal meningitis, but the risk of neurologic sequelae is much lower.

Feline spongiform encephalopathy (FSE) is a prion disease thought to be related or identical to Bovine spongiform encephalopathy (BSE).This disease is known to affect domestic and captive feline species. Lezmi S. et al. (2003), suggested that this infectious agent might be spread by both haematogenous and nervous pathways. Like BSE, this disease can take several years to develop. It is probable, but not proven, that the affected animals contract the disease by eating contaminated bovine meat.

Chronic Lyme disease is a generally unrecognised diagnosis that encompasses "a broad array of illnesses or symptom complexes for which there is no reproducible or convincing scientific evidence of any relationship to "B. burgdorferi" infection." There is no clinical evidence that "chronic" Lyme disease is caused by a persistent infection. It is distinct from post-treatment Lyme disease syndrome, a set of lingering symptoms which may persist after successful treatment of infection with Lyme spirochetes. The symptoms of "chronic Lyme" are generic and non-specific "symptoms of life".

A number of alternative treatments are promoted for "chronic Lyme disease", of which possibly the most controversial and harmful is long-term antibiotic therapy, particularly intravenous antibiotics. Most medical authorities advise against long-term antibiotic treatment for Lyme disease, though they agree that some patients do experience lingering symptoms. Following disciplinary proceedings by State medical licensing boards in the United States, a subculture of "Lyme literate" physicians has successfully lobbied for specific legal protections, exempting them from the standard of care and Infectious Diseases Society of America treatment guidelines. This "troubling" political interference in medical care has been criticised as an example of "legislative alchemy", the process whereby pseudomedicine is legislated into practice.

The disease appears to be progressive in nature. The Fields twins started having problems when they were four years old. By the time they had reached the age of nine, they were having difficulty walking and needed frames to assist them with walking. Their muscles have been gradually deteriorating over time. The disease affects the twins' nerves, causing them to make involuntary muscle movements such as trembling in the hands.

The extent of the disease is still unknown as the two women are only 21. However, the disease has had no apparent effect on their brains or personalities. Doctors do not know if the disease is fatal and, if so, what the life expectancy of one with this disease is. If the cause of the disease is genetic, there is a chance that the twins could pass it on to their future children.

The patient with meningococcal meningitis typically presents with high fever, nuchal rigidity (stiff neck), Kernig's sign, severe headache, vomiting, purpura, photophobia, and sometimes chills, altered mental status, or seizures. Diarrhea or respiratory symptoms are less common. Petechiae are often also present, but do not always occur, so their absence should not be used against the diagnosis of meningococcal disease. Anyone with symptoms of meningococcal meningitis should receive intravenous antibiotics before the results of lumbar puncture, as delay in treatment worsens the prognosis.

Although the dermatological changes are the most obvious symptoms of Urbach–Wiethe disease, many patients also have neurological symptoms. About 50–75% of the diagnosed cases of Urbach–Wiethe disease also show bilateral symmetrical calcifications on the medial temporal lobes. These calcifications often affect the amygdala and the periamygdaloid gyri. The amygdala is thought to be involved in processing biologically relevant stimuli and in emotional long term memory, particularly those associated with fear, and both PET and MRI scans have shown a correlation between amygdala activation and episodic memory for strongly emotional stimuli. Therefore, Urbach–Wiethe disease patients with calcifications and lesions in these regions may suffer impairments in these systems. These calcifications are the result of a buildup of calcium deposits in the blood vessels within this brain region. Over time, these vessels harden and the tissue they are a part of dies, causing lesions. The amount of calcification is often related to disease duration. The true prevalence of these calcifications is difficult to accurately state as not all patients undergo brain imaging. Some patients also exhibit epilepsy and neuropsychiatric abnormalities. Epilepsy symptoms could begin with light anxiety attacks and it can be controlled with "Epilum" (Epilepsy Medicine) Other patients present with symptoms similar to schizophrenia while some suffer from mood, anxiety, and psychotic disorders.

Although symptoms can vary greatly between affected individuals, even those within the same family, symptoms normally begin in infancy and are typically a result of thickening skin and mucous membranes. The first symptom is often a weak cry or a hoarse voice due to a thickening of the vocal cords. The hoarse voice can be one of the most striking clinical manifestations of the disease. Lesions and scars also appear on the skin, usually the face and the distal parts of the limbs. This is often the result of poor wound healing and the scarring continues to increase as the patient ages, leaving the skin with a waxy appearance. Skin may be easily damaged as a result of only a minor trauma or injury, leaving many blisters and additional scars. The skin is also usually very dry and wrinkly. White or yellow infiltrates form on the lips, buccal mucosa, tonsils, uvula, epiglottis and frenulum of the tongue. This can lead to upper respiratory tract infection and sometimes requires tracheostomy to relieve the symptom. Too much thickening of the frenulum can restrict tongue movement and may result in speech impediments. Beading of the papules around the eyelids is a very common symptom and is often used as part of a diagnosis of the disease. Some other dermatological symptoms that are sometimes seen but less common include hair loss, parotitis and other dental abnormalities, corneal ulceration, and focal degeneration of the macula.

Fields' disease is considered to be one of the rarest known diseases in the world, with only two diagnosed cases in history. The frequency of this disease is therefore 1 in approximately 3.75 billion (although since the disease manifested in identical twins, the actual frequency is 1 in approximately 7.5 billion). It is named after Welsh twins Catherine and Kirstie Fields, of Llanelli. Fields' disease is a neuromuscular disease, causing muscular degeneration.

The disease was first noticed when the twins were around the age of four. Doctors have been unable to identify it and have not been able to match it to any known diseases. As a result, the Fields sisters have undergone numerous tests, but no treatment has yet been found. No definitive cause has been determined and doctors have generally concluded that they were born with it.