Cryptosporidiosis may occur as an asymptomatic infection, an acute infection (i.e., duration shorter than 2 weeks), as recurrent acute infections in which symptoms reappear following a brief period of recovery for up to 30 days, and as a chronic infection (i.e., duration longer than 2 weeks) in which symptoms are severe and persistent. It may be fatal in individuals with a severely compromised immune system. Symptoms usually appear 5–10 days after infection (range: 2–28 days) and normally last for up to 2 weeks in immunocompetent individuals; symptoms are usually more severe and persist longer in immunocompromised individuals. Following the resolution of diarrhea, symptoms can reoccur after several days or weeks due to reinfection. Based upon one clinical trial, the likelihood of is high in immunocompetent adults.

In immunocompetent individuals, cryptosporidiosis is primarily localized to the distal small intestine and sometimes the respiratory tract as well. In immunocompromised persons, cryptosporidiosis may disseminate to other organs, including the hepatobiliary system, pancreas, upper gastrointestinal tract, and urinary bladder; pancreatic and biliary infection can involve acalculous cholecystitis, sclerosing cholangitis, papillary stenosis, or pancreatitis.

Cryptosporidiosis, also known as crypto, is a parasitic disease caused by "Cryptosporidium", a genus of protozoan parasites in the phylum Apicomplexa. It affects the distal small intestine and can affect the respiratory tract in both immunocompetent (i.e., individuals with a normal functioning immune system) and immunocompromised (e.g., persons with HIV/AIDS or autoimmune disorders) individuals, resulting in watery diarrhea with or without an unexplained cough. In immunocompromised individuals, the symptoms are particularly severe and can be fatal. It is primarily spread through the fecal-oral route, often through contaminated water; recent evidence suggests that it can also be transmitted via fomites in respiratory secretions.

"Cryptosporidium" is the organism most commonly isolated in HIV-positive patients presenting with diarrhea. Despite not being identified until 1976, it is one of the most common waterborne diseases and is found worldwide. The parasite is transmitted by environmentally hardy microbial cysts (oocysts) that, once ingested, sporozoites within oocysts excyst (i.e., are released) and result in an infection of intestinal epithelial tissue.

Coccidiosis is a parasitic disease of the intestinal tract of animals caused by coccidian protozoa. The disease spreads from one animal to another by contact with infected feces or ingestion of infected tissue. Diarrhea, which may become bloody in severe cases, is the primary symptom. Most animals infected with coccidia are asymptomatic, but young or immunocompromised animals may suffer severe symptoms and death.

While coccidia can infect a wide variety of animals, including humans, birds, and livestock, they are usually species-specific. One well-known exception is toxoplasmosis caused by "Toxoplasma gondii".

Humans may first encounter coccidia when they acquire a puppy or kitten that is infected. Other than "T. gondii", the infectious organisms are canine and feline-specific and are not contagious to humans, unlike the zoonotic diseases.

Avian malaria is a parasitic disease of birds, caused by parasite species belonging to the genera "Plasmodium" and "Hemoproteus" (phylum Apicomplexa, class Haemosporidia, family Plasmoiidae). The disease is transmitted by a dipteran vector including mosquitoes in the case of "Plasmodium" parasites and biting midges for "Hemoproteus." The range of symptoms and effects of the parasite on its bird hosts is very wide, from asymptomatic cases to drastic population declines due to the disease, as is the case of the Hawaiian honeycreepers. The diversity of parasites is large, as it is estimated that there are approximately as many parasites as there are species of hosts. Co-speciation and host switching events have contributed to the broad range of hosts that these parasites can infect, causing avian malaria to be a widespread global disease, found everywhere except Antarctica.

Half of all children and a quarter of previously healthy adults are asymptomatic with "Babesia" infection. When people do develop symptoms, the most common are fever and hemolytic anemia, symptoms that are similar to those of malaria. People with symptoms usually become ill 1 to 4 weeks after the bite, or 1 to 9 weeks after transfusion of contaminated blood products. A person infected with babesiosis gradually develops malaise and fatigue, followed by a fever. Hemolytic anemia, in which red blood cells are destroyed and removed from the blood, also develops. Chills, sweats, and thrombocytopenia are also common symptoms. Symptoms may last from several days to several months.

Less common symptoms and physical exam findings of mild-to-moderate babesiosis:

In more severe cases, symptoms similar to malaria occur, with fevers up to 40.5 °C (105 °F), shaking chills, and severe anemia (hemolytic anemia). Organ failure may follow, including adult respiratory distress syndrome. Severe cases occur mostly in people who have had a splenectomy. Severe cases are also more likely to occur in the very young, very old, and persons with immunodeficiency, such as HIV/AIDS patients.

A reported increase in human babesiosis diagnoses in the 2000s is thought to be caused by more widespread testing and higher numbers of people with immunodeficiencies coming in contact with ticks, the disease vector. Little is known about the occurrence of "Babesia" species in malaria-endemic areas, where "Babesia" can easily be misdiagnosed as "Plasmodium". Human patients with repeat babesiosis infection may exhibit premunity.

Avian malaria is most notably caused by Plasmodium relictum, a protist that infects birds in all parts of the world apart from Antarctica. There are several other species of "Plasmodium" that infect birds, such as "Plasmodium anasum" and "Plasmodium gallinaceum", but these are of less importance except, in occasional cases, for the poultry industry. The disease is found worldwide, with important exceptions. Usually, it does not kill birds. However, in areas where avian malaria is newly introduced, such as the islands of Hawaiʻi, it can be devastating to birds that have lost evolutionary resistance over time.

Feline zoonosis are the viral, bacterial, fungal, protozoan, nematode and arthropod infections that can be transmitted to humans from the domesticated cat, "Felis catus". Some of these are diseases are reemerging and newly emerging infections or infestations caused by zoonotic pathogens transmitted by cats. In some instances, the cat can display symptoms of infection (these may differ from the symptoms in humans) and sometimes the cat remains asymptomatic. There can be serious illnesses and clinical manifestations in people who become infected. This is dependent on the immune status and age of the person. Those who live in close association with cats are more prone to these infections. But those that do not keep cats as pets are also able to acquire these infections because of the transmission can be from cat feces and the parasites that leave their bodies.

People can acquire cat-associated infections through bites, scratches or other direct contact of the skin or mucous membranes with the cat. This includes 'kissing' or letting the animal lick the mouth or nose. Mucous membranes are easily infected when the pathogen is in the mouth of the cat. Pathogens can also infect people when there is contact with animal saliva, urine and other body fluids or secretions, When fecal material is unintentionally ingested, infection can occur. Feline zooinosis can be acquired by a person by inhalation of aerosols or droplets coughed up by the cat.

In the United States, forty percent of homes have at least one cat. Some contagious infections such as campylobacteriosis and salmonellosis cause visible symptoms of the disease in cats. Other infections, such as cat scratch disease and toxoplasmosis, have no visible symptoms and are carried by apparently healthy cats.

Babesiosis is a malaria-like parasitic disease caused by infection with "Babesia", a genus of Apicomplexa. Human babesiosis is an uncommon but emerging disease in the Northeastern and Midwestern United States and parts of Europe, and sporadic throughout the rest of the world. It occurs in warm weather. Ticks transmit the human strain of babesiosis, so it often presents with other tick-borne illnesses such as Lyme disease. After trypanosomes, "Babesia" is thought to be the second-most common blood parasite of mammals, and they can have a major impact on health of domestic animals in areas without severe winters. In cattle, a major host, the disease is known as Texas cattle fever, redwater, or piroplasmosis.

Puppies are frequently infected with coccidia from the feces of their mother, and are more likely to develop coccidiosis due to their undeveloped immune systems. Stress can trigger symptoms in susceptible animals.

Symptoms in young dogs include diarrhea with mucus and blood, poor appetite, vomiting, and dehydration. Untreated the disease can be fatal.

Treatment is routine and effective. Diagnosis is made by low-powered microscopic examination of the feces, which is generally replete with oocysts. Readily available drugs eliminate the protozoa or reduce them enough that the animal's immune system can clear the infection. Permanent damage to the gastrointestinal system is rare, and a dog will usually suffer no long-lasting negative effects.

Cryptosporidiosis is a parasitic disease that is transmitted through contaminated food or water from an infected person or animal. Cryptosporidiosis in cats is rare, but they can carry the protozoan without showing any signs of illness. Cryptosporidiosis can cause profuse, watery diarrhea with cramping, abdominal pain, and nausea in people. Illness in people is usually self-limiting and lasts only 2–4 days, but can become severe in people with weakened immune systems. Cryptosporidiosis (Cryptosporidium spp.) Cats transmit the protozoan through their feces. The symptoms in people weight loss and chronic diarrhea in high-risk patients. More than one species of this genus can be acquired by people. Dogs can also transmit this parasite.

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".

Eustrongylidosis is a parasitic disease that mainly affects wading birds worldwide; however, the parasite’s complex, indirect life cycle involves other species such as aquatic worms and fish. Moreover, this disease is zoonotic which means the parasite can transmit disease from animals to humans. Eustrongylidosis is named after the causative agent Eustrongylides and typically occurs in eutrophicated waters where concentrations of nutrients and minerals are high enough to provide ideal conditions for the parasite to thrive and persist. Because eutrophication has become a common issue due to agricultural runoff and urban development, cases of Eustrongylidosis are becoming prevalent and hard to control. Eustrongylidosis can be diagnosed before or after death by observing behavior, clinical signs and performing fecal flotations and necropsies. Methods to control Eustrongylidosis include preventing eutrophication and providing hosts with uninfected food sources in aquaculture farms. Parasites are known to be indicators of environmental health and stability and should therefore be studied further to better understand the parasite’s life cycle and how it affects predator-prey interactions and improve conservation efforts.

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.

Histomoniasis (or histomonosis), also known as blackhead disease, is a commercially important disease of poultry, particularly of chickens and turkeys, due to parasitic infection of a protozoan, "Histomonas meleagridis". The protozoan is transmitted to the bird by the nematode parasite "Heterakis gallinarum". "H. meleagridis" resides within the eggs of "H. gallinarum", so birds ingest the parasites along with contaminated soil or food. Earthworms can also act as a paratenic host.

"Histomonas meleagridis" specifically infects the cecum and liver. Symptoms of the infection include depression, reduced appetite, poor growth, increased thirst, sulphur-yellow diarrhoea, listlessness, and dry, ruffled feathers. The head may become cyanotic (bluish in colour), hence the common name of the disease, blackhead disease; thus the name 'blackhead' is in all possibility a misnomer for discoloration. The disease carries a high mortality rate, and is particularly highly fatal in poultry, and less in other birds. Currently, no prescription drug is available to treat this disease.

Poultry (especially free-ranging) and wild birds commonly harbor a number of parasitic worms with only mild health problems from them. Turkeys are much more susceptible to getting blackhead than are chickens. Thus, chickens can be infected carriers for a long time because they are not removed or medicated by their owners, and they do not die or stop eating/defecating. "H. gallinarum" eggs can remain infective in soil for four years, a high risk of transmitting blackhead to turkeys remains if they graze areas with chicken feces in this time frame.

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.).

Histomoniasis is characterized by blackhead in birds. "H. meleagridis" is released in the cecum where the eggs of the nematode undergo larval development. The parasite migrates to the mucosa and submucosa where they cause extensive and severe necrosis of the tissue. Necrosis is initiated by inflammation and gradual ulceration, causing thickening of the cecal wall. The lesions are sometimes exacerbated by other pathogens such as "Escherichia coli" and coccidia. Histomonads then gain entry into small veins of the blood stream from the cecal lesions and migrate to the liver, causing focal necrosis. Turkeys are noted to be most susceptible to the symptoms in terms of mortality, sometimes approaching 100% of a flock. Diagnosis can be easily performed by necropsy of the fresh or preserved carcass. Unusual lesions have been observed in other organs of turkey such as the bursa of Fabricius, lungs, and kidneys.

Symptoms appear within 7–12 days after infection and include depression, reduced appetite, poor growth, increased thirst, sulphur-yellow diarrhoea, listlessness, drooping wings, and unkempt feathers. Young birds have a more acute disease and die within a few days after signs appear. Older birds may be sick for some time and become emaciated before death. The symptoms are highly fatal to turkeys, but effect less damage in chickens. However, outbreaks in chickens may result in high morbidity, moderate mortality, and extensive culling, leading to overall poor flock performance. Concurrence of "Salmonella typhmurium" and "E. coli" was found to cause high mortality in broiler chickens.

The acute form of the disease is manifested by lethargy, loss of appetite, vomiting and diarrhea. Due to the severe immune system suppression, multiple secondary infections develop, causing death within two to four weeks. Typical confirmation of the acute form of the disease is by necropsy, because it progresses too quickly for the normal signs such as feather loss and beak deformity to appear.

The chronic form occurs if the bird's immune system manages to mount a reaction to the virus and any secondary infections. The characteristic feather symptoms need time to develop, as they only appear during the first moult after infection. In those species having powder down, signs may be visible immediately, as powder down feathers are continually replenished.

Avian Botulism is a strain of botulism that affects wild and captive bird populations, most notably waterfowl. This is a paralytic disease brought on by the Botulinum neurotoxin (BoNt) of the bacterium "Clostridium botulinum". "C. botulinum" can fall into one of 7 different types which are strains A through G. Type C BoNt is most frequently associated with waterfowl mortality. The Type E strain is also commonly associated with avian outbreaks and is frequently found in fish species which is why most outbreaks occur in piscivorous birds.

Avian Botulism occurs all over the world and its understanding is important for wildlife managers, hunters, bird watchers, and anyone who owns wetland property as this disease can account for over 1,000,000 waterbird deaths in a year.

Psittacine beak and feather disease (PBFD) is a viral disease affecting all Old World and New World parrots. The causative virus–beak and feather disease virus (BFDV)—belongs to the taxonomic genus Circovirus, family Circoviridae. It attacks the feather follicles and the beak and claw matrices of the bird, causing progressive feather, claw and beak malformation and necrosis. In later stages of the disease, feather shaft constriction occurs, hampering development until eventually all feather growth stops. It occurs in an acutely fatal form and a chronic form.

Cracking and peeling of the outer layers of the claws and beak make tissues vulnerable to . Because the virus also affects the thymus and Bursa of Fabricius, slowing lymphocyte production, immunosuppression occurs and the bird becomes more vulnerable to secondary infections. Beak fractures and necrosis of the hard palate can prevent the bird from eating.

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.

The incubation period for WNV—the amount of time from infection to symptom onset—is typically from between 2 and 15 days. Headache can be a prominent symptom of WNV fever, meningitis, encephalitis, meningoencephalitis, and it may or may not be present in poliomyelitis-like syndrome. Thus, headache is not a useful indicator of neuroinvasive disease.

- West Nile fever (WNF), which occurs in 20 percent of cases, is a febrile syndrome that causes flu-like symptoms. Most characterizations of WNF generally describe it as a mild, acute syndrome lasting 3 to 6 days after symptom onset. Systematic follow-up studies of patients with WNF have not been done, so this information is largely anecdotal. In addition to a high fever, headache, chills, excessive sweating, weakness, fatigue, swollen lymph nodes, drowsiness, pain in the joints and flu-like symptoms. Gastrointestinal symptoms that may occur include nausea, vomiting, loss of appetite, and diarrhea. Fewer than one-third of patients develop a rash.

- West Nile neuroinvasive disease (WNND), which occurs in less than 1 percent of cases, is when the virus infects the central nervous system resulting in meningitis, encephalitis, meningoencephalitis or a poliomyelitis-like syndrome. Many patients with WNND have normal neuroimaging studies, although abnormalities may be present in various cerebral areas including the basal ganglia, thalamus, cerebellum, and brainstem.

- West Nile virus encephalitis (WNE) is the most common neuroinvasive manifestation of WNND. WNE presents with similar symptoms to other viral encephalitis with fever, headaches, and altered mental status. A prominent finding in WNE is muscular weakness (30 to 50 percent of patients with encephalitis), often with lower motor neuron symptoms, flaccid paralysis, and hyporeflexia with no sensory abnormalities.

- West Nile meningitis (WNM) usually involves fever, headache, and stiff neck. Pleocytosis, an increase of white blood cells in cerebrospinal fluid, is also present. Changes in consciousness are not usually seen and are mild when present.

- West Nile meningoencephalitis is inflammation of both the brain (encephalitis) and meninges (meningitis).

- West Nile poliomyelitis (WNP), an acute flaccid paralysis syndrome associated with WNV infection, is less common than WNM or WNE. This syndrome is generally characterized by the acute onset of asymmetric limb weakness or paralysis in the absence of sensory loss. Pain sometimes precedes the paralysis. The paralysis can occur in the absence of fever, headache, or other common symptoms associated with WNV infection. Involvement of respiratory muscles, leading to acute respiratory failure, can sometimes occur.

- West-Nile reversible paralysis, Like WNP, the weakness or paralysis is asymmetric. Reported cases have been noted to have an initial preservation of deep tendon reflexes, which is not expected for a pure anterior horn involvement. Disconnect of upper motor neuron influences on the anterior horn cells possibly by myelitis or glutamate excitotoxicity have been suggested as mechanisms. The prognosis for recovery is excellent.

- Nonneurologic complications of WNV infection that may rarely occur include fulminant hepatitis, pancreatitis, myocarditis, rhabdomyolysis, orchitis, nephritis, optic neuritis and cardiac dysrhythmias and hemorrhagic fever with coagulopathy. Chorioretinitis may also be more common than previously thought.

- Cutaneous manifestations specifically rashes, are not uncommon in WNV-infected patients; however, there is a paucity of detailed descriptions in case reports and there are few clinical images widely available. Punctate erythematous, macular, and papular eruptions, most pronounced on the extremities have been observed in WNV cases and in some cases histopathologic findings have shown a sparse superficial perivascular lymphocytic infiltrate, a manifestation commonly seen in viral exanthems. A literature review provides support that this punctate rash is a common cutaneous presentation of WNV infection.

Eastern equine encephalitis (EEE), commonly called Triple E or, sleeping sickness (not to be confused with "Trypanosomiasis") is a zoonotic alphavirus and arbovirus present in North, Central and South America and the Caribbean. EEE was first recognized in Massachusetts, United States in 1831 when 75 horses died mysteriously of viral encephalitis.

Epizootics in horses have continued to occur regularly in the United States. It can also be identified in asses and zebras. Due to the rarity of the disease its occurrence can cause economic impact in relation to the loss of horses and poultry. EEE is found today in the eastern part of the country and is often associated with coastal plains. It can most commonly be found in east and gulf coast states. In Florida about one to two human cases are reported a year although over sixty cases of equine encephalitis are reported. Some years in which there are favorable conditions for the disease there number of equine cases are over two-hundred. Diagnosing equine encephalitis is challenging because many of the symptoms are shared with other illnesses and patients can be asymptomatic. Confirmations may require a sample of cerebral spinal fluid or brain tissue although CT scans and MRI scans are used to detect encephalitis. This could be an indication that the need to test for Eastern Equine Encephalitis is necessary. If a biopsy of the cerebral spinal fluid is taken it is sent to a specialized laboratory for testing.

EEEV is closely related to Venezuelan equine encephalitis virus and Western equine encephalitis virus.

Diagnosis in a live specimen is possible in the field by palpating the abdomen. As with birds, prominence of the keel could be a determinant in diagnosis, but natural history of the species needs to be understood to avoid potential misdiagnoses. However, the best form of diagnosis still remains as necropsy. During the necropsy, the best diagnosis can be determined by the adult nematodes by scanning them with electron microscopy. Different species of Eustrongylidosis nematodes can be differentiated by specific gender characteristics, i.e. “Male specimens of E. ignotus have a caudal sucker that lacks cuticular cleft, while a cuticular cleft is present in the caudal sucker of male specimens of E. excisus”. “Eustrongylidosis can often be misdiagnosed as starvation in nestling because they are often emaciated at the time of death”.

Before necropsy takes place, diagnosis by palpitation can be used to find tubular lesions. Those tubular lesions are firm, firmly attached to organs, and are felt in the subcutaneous tissue. While palpitation is practical and simple, errors can be made in nestlings’ examinations because their ribs have the potential to present as lesions. Diagnosis is also attainable by examining fecal samples, but has the high potential of false negatives. That possibility is increased in fledging feces “where severe disease may precede appearance of eggs in the feces”.

In humans, after an incubation period of 5–19 days, the symptoms of the disease range from inapparent illness to systemic illness with severe pneumonia. It presents chiefly as an atypical pneumonia. In the first week of psittacosis the symptoms mimic typhoid fever: prostrating high fevers, joint pains, diarrhea, conjunctivitis, nose bleeds and low level of white blood cells in the blood. Rose spots can appear and these are called Horder's spots. Spleen enlargement is common towards the end of the first week. It may become a serious lung infection. Diagnosis can be suspected in case of respiratory infection associated with splenomegaly and/or epistaxis. Headache can be so severe that it suggests meningitis and some nuchal rigidity is not unusual. Towards the end of the first week stupor or even coma can result in severe cases.

The second week is more akin to acute bacteremic pneumococcal pneumonia with continuous high fevers, headaches, cough, and dyspnea. X-rays show patchy infiltrates or a diffuse whiteout of lung fields.

Complications in the form of endocarditis, liver inflammation, inflammation of the heart's muscle, joint inflammation, keratoconjunctivitis (occasionally extranodal marginal zone lymphoma of the lacrimal gland/orbit), and neurologic complications (brain inflammation) may occasionally occur. Severe pneumonia requiring intensive-care support may also occur. Fatal cases have been reported (less than 1% of cases).

Cats with Avian Influenza exhibit symptoms that can result in death. They are one of the few species that can get Avian Influenza. The specific virus that they get is H5N1, which is a subtype of Avian Influenza. In order to get the virus, cats need to be in contact with waterfowl, poultry, or uncooked poultry that are infected. Two of the main organs that the virus affects are the lungs and liver.