A slow virus is a virus, or a viruslike agent, etiologically associated with a disease, having a long incubation period of months to years and then a gradual onset of symptoms which progress slowly but irreversibly and terminate in a severe compromised state or, more commonly, death.

A slow virus disease is a disease that, after an extended period of latency, follows a slow, progressive course spanning months to years, frequently involving the central nervous system and ultimately leading to death. Examples include the Visna-Maedi virus, in the genus Lentivirus (family Retroviridae), that causes encephalitis and chronic pneumonitis in sheep, and subacute sclerosing panencephalitis which is apparently caused by the measles virus, as well as Paget's Disease of Bone (Osteitis Deformans) which is associated with paramyxoviridae, especially RSV and Rubeola (Measles).

The clinical presentation of prion diseases will vary from patient to patient. However, some general characteristics of prion diseases are listed below.

The signs shown depend on the horse's age, the strain of the infecting virus, the condition of the horse and the route by which it was infected. Most horses with EVA infection don't show any signs; if a horse does show symptoms, these can vary greatly in severity. Following infection, the first sign is fever, peaking at , followed by various signs such as depression, nasal discharge, "pink eye" (conjunctivitis), swelling over the eye (supraorbital edema), urticaria, and swelling of the limbs and under the belly (the ventral abdomen) which may extend to the udder in mares or the scrotum of male horses. More unusual signs include abortion in pregnant mares, and, most likely in foals, severe respiratory distress and death.

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.

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.

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.

Signs and symptoms may vary and some individuals may not experience any symptoms at all. The most common reported symptom of polioencephalitis is fatigue. Fatigue is associated with difficulty in attention, cognition, and maintaining wakefulness Some individuals experience psychiatric symptoms that include anxious mood, pain, insomnia, and depressed mood. Confusion and disorientation of time and space have also been reported. Motor symptoms vary more from patient to patient, but can include incoordination and tremors, nystagmus, loss of conjugate eye movements, rigidity and hemiparesis.

Bluetongue disease is a non-contagious, insect-borne, viral disease of ruminants, mainly sheep and less frequently cattle, goats, buffalo, deer, dromedaries, and antelope. It is caused by the Bluetongue virus (BTV). The virus is transmitted by the midge "Culicoides imicola", "Culicoides variipennis", and other culicoids.

Equine viral arteritis (EVA) is a disease of horses caused by equine arteritis virus, an RNA virus of the genus "Arterivirus". The virus which causes EVA was first isolated in 1953, but the disease has afflicted equine animals worldwide for centuries. It has been more common in some breeds of horses in the United States, but there is no breed "immunity". In the UK, it is a notifiable disease. There is no known human hazard.

Foot-and-mouth disease or hoof-and-mouth disease (Aphthae epizooticae) is an infectious and sometimes fatal viral disease that affects cloven-hoofed animals, including domestic and wild bovids. The virus causes a high fever for approximately two to six days, followed by blisters inside the mouth and on the feet that may rupture and cause lameness.

Foot-and-mouth disease (FMD) has very severe implications for animal farming, since it is highly infectious and can be spread by infected animals comparatively easily through contact with contaminated farming equipment, vehicles, clothing, feed and by domestic and wild predators. Its containment demands considerable efforts in vaccination, strict monitoring, trade restrictions, quarantines and occasionally the culling of animals.

Susceptible animals include cattle, water buffalo, sheep, goats, pigs, antelope, deer, and bison. It has also been known to infect hedgehogs and elephants; llamas and alpacas may develop mild symptoms, but are resistant to the disease and do not pass it on to others of the same species. In laboratory experiments, mice, rats, and chickens have been successfully infected by artificial means, but they are not believed to contract the disease under natural conditions. Humans are very rarely infected.

The virus responsible for the disease is a picornavirus, the prototypic member of the genus "Aphthovirus". Infection occurs when the virus particle is taken into a cell of the host. The cell is then forced to manufacture thousands of copies of the virus, and eventually bursts, releasing the new particles in the blood. The virus is genetically highly variable, which limits the effectiveness of vaccination.

Clinical symptoms of viral infection include external hemorrhaging, pale gills, and ascites. In some cases, mortality can occur without any apparent clinical signs of the disease. The virus has been found in high concentrations in the liver and kidney, but lower numbers of virions have been isolated from the spleen. The virus has been shown to persist subclinically in fish populations up to 10 weeks following experimental infection. Currently efforts have been made to prevent infection by the virus through the development of DNA vaccines and immunostimulatory therapeutics.

Spring viraemia of carp, also known as Swim Bladder Inflammation, is caused by a rhabdovirus called "Rhabdovirus carpio". It is listed as a notifiable disease under the World Organisation for Animal Health.

Paravaccinia virus presents itself with blisters, nodules, or lesions about 4 mm in diameter, typically in the area that has made contact with livestock that is infected with bovine papular stomatitis. Lesions may begin forming as late as three weeks after contact has been made with an infected animal. In rare cases, lesions may be seen systemic. General signs of infection are also common, such as fever and fatigue.

Infected livestock may present with blisters or lesions on their udders or snout. Often, however, infected livestock show little to no symptoms.

Symptoms can develop over several weeks to months. Symptoms depend on location of damage in the brain and the degree of damage. The most prominent symptoms are "clumsiness, progressive weakness and visual, speech, and sometimes personality changes"

The lesions affecting the parietal and occipital lobes can lead to a phenomenon known as alien hand syndrome.

The incubation period for foot-and-mouth disease virus has a range between one and 12 days. The disease is characterized by high fever that declines rapidly after two or three days, blisters inside the mouth that lead to excessive secretion of stringy or foamy saliva and to drooling, and blisters on the feet that may rupture and cause lameness. Adult animals may suffer weight loss from which they do not recover for several months, as well as swelling in the testicles of mature males, and in cows, milk production can decline significantly. Though most animals eventually recover from FMD, the disease can lead to myocarditis (inflammation of the heart muscle) and death, especially in newborn animals. Some infected ruminants remain asymptomatic carriers, but they nonetheless carry FMDV and may be able to transmit it to others. Pigs cannot serve as asymptomatic carriers.

Polioencephalitis is a viral infection of the brain, causing inflammation within the grey matter of the brain stem. The virus has an affinity for neuronal cell bodies and has been found to affect mostly the midbrain, pons, medulla and cerebellum of most infected patients. The infection can reach up through the thalamus and hypothalamus and possibly reach the cerebral hemispheres. The infection is caused by the poliomyelitis virus which is a single-stranded RNA virus surrounded by a non-enveloped capsid. Humans are the only known natural hosts of this virus. The disease has been eliminated from the U.S. since the mid-twentieth century, but is still found in certain areas of the world such as Africa.

Mayaro virus disease is a mosquitoborne zoonotic pathogen endemic to certain humid forests of tropical South America. Infection with Mayaro virus causes an acute, self-limited dengue-like illness of 3–5 days' duration. The causative virus, abbreviated MAYV, is in the family Togaviridae, and genus Alphavirus. It is closely related to other alphaviruses that produce a dengue-like illness accompanied by long-lasting arthralgia. It is only known to circulate in tropical South America.

The majority of infections result in mild illness, including fever and headache. When infection is more severe the person may experience headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions and spastic paralysis. Fatality ranges from . Aged people are more likely to have a fatal infection.

Progressive multifocal leukoencephalopathy (PML) is a rare and usually fatal viral disease characterized by progressive damage ("-pathy") or inflammation of the white matter ("leuko-") of the brain ("-encephalo-") at multiple locations ("multifocal"). It is caused by the JC virus, which is normally present and kept under control by the immune system. JC virus is harmless except in cases of weakened immune systems. In general, PML has a mortality rate of 30–50 percent in the first few months and those who survive can be left with varying degrees of neurological disabilities.

PML occurs almost exclusively in patients with severe immune deficiency, most commonly among patients with acquired immune deficiency syndrome (AIDS), but people on chronic immunosuppressive medications including chemotherapy are also at increased risk of PML, such as patients with transplants, Hodgkin's lymphoma, multiple sclerosis, psoriasis and other autoimmune diseases.

Oropouche fever is characterized as a acute febrile illness, meaning that it begins with a sudden onset of a fever followed by severe clinical symptoms. It typically takes 4 to 8 days from the incubation period to first start noticing signs of infection, beginning from the bite of the infected mosquito or midge.

Fevers are the most common symptom with temperatures as high as 104F. Clinical symptoms include chills, headache, myalgia, arthralgia, dizziness, photophobia, vomiting, joint pains, epigastric pain, and rashes.

There also have been some cases where rashes resembles rubella and patients presented systematic symptoms including nausea, vomiting, diarrhea, conjunctive congestion, epigastric pain, and retro-orbitial pain.

The initial febrile episode typically passes after a few days, but it is very common to have a reoccurrence of these symptoms with a lesser intensity. Studies have shown this typically happens in about 60% of cases.

Paravaccinia virus is a viral infection of the Parapoxvirus genus of viruses. Human can contract the virus from contact with livestock infected with Bovine papular stomatitis and is common with ranchers, milkers, and veterinarians. Infection will present with fever, fatigue, and lesion on the skin.

Saint Louis encephalitis is a disease caused by the mosquito borne Saint Louis encephalitis virus. Saint Louis encephalitis virus is related to Japanese encephalitis virus and is a member of the Flaviviridae subgroup. This disease mainly affects the United States. Occasional cases have been reported from Canada and Mexico.

Rocio viral encephalitis is an epidemic flaviviral disease of humans first observed in São Paulo State, Brazil, in 1975. Low-level enzootic transmission is likely continuing in the epidemic zone, and with increased deforestation and population expansion, additional epidemics caused by Rocio virus are highly probable. If migratory species of birds are, or become involved in, the virus transmission cycle, the competency of a wide variety of mosquito species for transmitting Rocio virus experimentally suggest that the virus may become more widely distributed. The encephalitis outbreak in the western hemisphere caused by West Nile virus, a related flavivirus, highlights the potential for arboviruses to cause severe problems far from their source enzootic foci.

The causative Rocio virus belongs to the genus "Flavivirus" (the same genus as the Zika virus) in family Flaviviridae and is closely related serologically to Ilhéus, St. Louis encephalitis, Japanese encephalitis and Murray Valley encephalitis viruses.

Lábrea fever has a sudden onset, with jaundice (yellowing of the skin), anorexia (lack of appetite), hematemesis (vomiting of blood), headache, fever and severe prostration. Death occurs by acute liver failure (ALF). In the last phase, neurological symptoms such as agitation, delirium, convulsions and hemorrhagic coma commonly appear.

Orf is an exanthemous disease caused by a parapox virus and occurring primarily in sheep and goats. It is also known as contagious pustular dermatitis, infectious labial dermatitis, ecthyma contagiosum, thistle disease and scabby mouth. "Orf virus" is zoonotic—it can also infect humans.