Paravaccinia virus originates from livestock infected with bovine papular stomatitis. When a human makes physical contact with the livestock's muzzle, udders, or an infected area, the area of contact will become infected. Livestock may not show symptoms of bovine papular stomatitis and still be infected and contagious. Paravaccinia can enter the body though all pathways including: skin contact by mechanical means, through the respiratory tract, or orally. Oral or respiratory contraction may be more likely to cause systemic symptoms such as lesions across the whole body

A person who has not previously been infected with paravaccinia virus should avoid contact with infected livestock to prevent contraction of disease. There is no commercially available vaccination for cattle or humans against paravaccinia. However, following infection, immunization has been noted in humans, making re-infection difficult. Unlike other pox viruses, there is no record of contracting paravaccinia virus from another human. Further, cattle only show a short immunization after initial infection, providing opportunity to continue to infect more livestock and new human hosts.

It has been recorded since the late 19th century and has been reported from most sheep-or goat-raising areas including those in Europe, the Middle East, the United States, Africa, Asia, Alaska, South America, Canada, New Zealand and Australia. Orf is spread by fomites and direct contact. In some environments infection is injected by scratches from thistles of both growing and felled plants. Symptoms include papules and pustules on the lips and muzzle, and less commonly in the mouth of young lambs and on the eyelids, feet, and teats of ewes. The lesions progress to thick crusts which may bleed. Orf in the mouths of lambs may prevent suckling and cause weight loss, and can infect the udder of the mother ewe, thus potentially leading to mastitis. Sheep are prone to reinfection. Occasionally the infection can be extensive and persistent if the animal does not produce an immune response.

A live virus vaccine (ATCvet code: ) is made from scab material and usually given to ewes at the age of two months, but only to lambs when there is an outbreak. The vaccine can cause disease in humans.

In sheep and goats the lesions mostly appear on or near the hairline and elsewhere on the lips and muzzle. In some cases the lesions appear on and in the nostrils, around the eyes, on the thigh, coronet, vulva, udder and axilla. In rare cases, mostly involving young lambs, lesions are found on the tongue, gums, roof of the mouth and the oesophagus. It has also been reported a number of times to cause lesions in the rumen. In one case it was shown that a severe form of orf virus caused an outbreak involving the gastrointestinal tract, lungs, heart, as well as the buccal cavity, cheeks, tongue and lips. Another severe case was reported pharyngitis, genital lesions and infection of the hooves which led to lameness and, in some cases, sloughing of the hoof.

More typically sheep will become free of orf within a week or so as the disease runs its course. Sheep custodians can assist by ensuring infected lambs receive sufficient milk and separating out the infected stock to slow down cross-transmission to healthy animals. It is advisable for those handling infected animals to wear disposable gloves to prevent cross-infection and self-infection. A veterinarian needs to be contacted if there is a risk of misdiagnosis with other, more serious conditions.

Primarily, orf is a disease of sheep and goats although it has been reported as a natural disease in the following: humans, steenbok and alpacas, chamois and thar, reindeer, musk ox, dog, cat, mountain goat, bighorn sheep, dall sheep, and the red squirrel .

A contagious disease is a subset category of transmissible diseases, which are transmitted to other persons, either by physical contact with the person suffering the disease, or by casual contact with their secretions or objects touched by them or airborne route among other routes.

Non-contagious infections, by contrast, usually require a special mode of transmission between persons or hosts. These include need for intermediate vector species (mosquitoes that carry malaria) or by non-casual transfer of bodily fluid (such as transfusions, needle sharing or sexual contact).

The boundary between contagious and non-contagious infectious diseases is not perfectly drawn, as illustrated classically by tuberculosis, which is clearly transmissible from person to person, but was not classically considered a contagious disease. In the present day, most sexually transmitted diseases are considered contagious, but only some of them are subject to medical isolation.

Variola caprina (goat pox) is a contagious viral disease caused by a pox virus that affects goats. The virus usually spreads via the respiratory system, and sometimes spreads through abraded skin. It is most likely to occur in crowded stock. Sources of the virus include cutaneous lesions, saliva, nasal secretions and faeces. There are two types of the disease: the papulo-vesicular form and the nodular form (stone pox). The incubation period is usually 8–13 days, but it may be as short as four days.

It is thought the same virus spreads sheep pox, to which European sheep breeds are highly susceptible. The virus may be present in dried scabs for up to six months.

In endemic areas the morbidity rate is 70–90% and the mortality rate is 5–10%. The mortality rate may reach nearly 100% in imported animals. Resistant animals may show only a mild form of the disease, which may be missed as only a few lesions are present, usually around the ears or the tail.

There is no vaccine for SVD. Prevention measures are similar to those for foot-and-mouth disease: controlling animals imported from infected areas, and sanitary disposal of garbage from international aircraft and ships, and thorough cooking of garbage. Infected animals should be placed in strict quarantine. Eradication measures for the disease include quarantining infected areas, depopulation and disposal of infected and contact pigs, and cleaning and disinfecting

contaminated premises.

Originally, the term referred as sometimes been broadened to encompass "any" communicable or infectious disease. Often the word can only be understood in context, where it is used to emphasise very infectious, easily transmitted, or especially severe communicable disease. They could be very dangerous.

Infectious diseases are sometimes called contagious disease when they are easily transmitted by contact with an ill person or their secretions (e.g., influenza). Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted. Other types of infectious/transmissible/communicable diseases with more specialized routes of infection, such as vector transmission or sexual transmission, are usually not regarded as "contagious", and often do not require medical isolation (sometimes loosely called quarantine) of victims. However, this specialized connotation of the word "contagious" and "contagious disease" (easy transmissibility) is not always respected in popular use.

Infectious diseases are commonly transmitted from person to person through direct contact. The types of contact are through person to person and droplet spread. Indirect contact such as airborne transmission, contaminated objects, food and drinking water, animal person contact, animal reservoirs, insect bites, and environmental reservoirs are another way infectious diseases are transmitted,

Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microorganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani releases a toxin that paralyzes muscles, and staphylococcus releases toxins that produce shock and sepsis. Not all infectious agents cause disease in all hosts. For example, less than 5% of individuals infected with polio develop disease. On the other hand, some infectious agents are highly virulent. The prion causing mad cow disease and Creutzfeldt–Jakob disease invariably kills all animals and people that are infected.

Persistent infections occur because the body is unable to clear the organism after the initial infection. Persistent infections are characterized by the continual presence of the infectious organism, often as latent infection with occasional recurrent relapses of active infection. There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise.

Persistent infections cause millions of deaths globally each year. Chronic infections by parasites account for a high morbidity and mortality in many underdeveloped countries.

Swine vesicular disease is most commonly brought into a herd by the introduction of a subclinically infected pig.

The disease can be transmitted in feed containing infected meat scraps, or by direct contact with infected feces (such as in an improperly cleaned truck).

Paravaccinia is a member of the Parapoxvirus family. It has a cylindrical body about 140 X 310 nm in size, with convex ends covered in a criss-cross pattern of rope like structures. The virus is resistant to cold, dehydration, and temperatures up to 56 °C. Upon injecting a cell with its genome, the virus begins transcription in the cytoplasm using viral RNA polymerase. As the virus progresses through the cell, the host begins to replicate the viral genome between 140 minutes and 48 hours.

Goat pox is found in the part of Africa north of the equator, the Middle East, Central Asia and India. It may be spread between animals by:

- Direct contact

- Indirect transmission by contaminated implements, vehicles or products such as litter or fodder

- Indirect transmission by insects (mechanical vectors).

- Contamination by inhalation, intradermal or subcutaneous inoculation, or by respiratory, transcutaneous and transmucosal routes

Contagious bovine pleuropneumonia (CBPP - also known as lung plague), is a contagious bacterial disease that afflicts the lungs of cattle, buffalo, zebu, and yaks.

It is caused by the bacterium "Mycoplasma mycoides", and the symptoms are pneumonia and inflammation of the lung membranes. The incubation period is 20 to 123 days. It was particularly widespread in the United States in 1879, affecting herds from several states. The outbreak was so severe that it resulted in a trade embargo by the British government, blocking U.S. cattle exports to Britain and Canada. This prompted the United States to establish the Bureau of Animal Industry, set up in 1884 to eradicate the disease, which it succeeded in doing by 1892.

Louis Willems, a Belgian doctor, began pioneering work in the 1850s on animal inoculation against the disease.

The bacteria are widespread in Africa, the Middle East, Southern Europe, as well as parts of Asia. It is an airborne species, and can travel up to several kilometres in the right conditions.

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.

Ultraviolet (UV) radiation is implicated in cattle with no pigmentation around the eyelids and cattle with prominently placed eyes. Exudate from the sun-burnt skin around the eyes can contain bacteria and attracts flies. UV light also directly damages the corneal epithelium, leading to a breakdown in host innate immunity.

Dust, dried-up plants, tall vegetation, and oversized or incorrectly placed ear tags may cause mechanical damage to the eye and facilitate bacterial colonization.

The disease may be complicated by concurrent infection with viruses such as infectious bovine rhinotracheitis virus (bovine herpesvirus 1) or adenovirus, bacteria such as "Mycoplasma boviculi" or "Listeria monocytogenes", or infestation by "Thelazia", a nematode.

Vitamin A deficiency is also implicated.

IBK is most prevalent in summer and early autumn.

A recent Meat and Livestock Australia report "estimates that the disease costs Australian beef producers AU$23.5 million annually in lost production and treatment costs".

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.

"Moraxella bovis" is a Gram-negative rod-shaped aerobe. This bacterium is an obligate intracellular parasite of the mucous membranes, and can usually be isolated from the respiratory tract, vagina, and conjunctiva of healthy animals. Transmission of IBK is through direct contact with mucous membranes and their secretions and indirect contact where flies act as a mechanical vector. Asymptomatic carrier animals can also be source of infection.

The duration of the visible blistering caused by varicella zoster virus varies in children usually from 4 to 7 days, and the appearance of new blisters begins to subside after the fifth day. Chickenpox infection is milder in young children, and symptomatic treatment, with sodium bicarbonate baths or antihistamine medication may ease itching. It is recommended to keep new infants from birth up to age 6 months away from an infected person for 10 to 21 days because their immune systems are not developed enough to handle the stress it can bring on. Paracetamol (acetaminophen) is widely used to reduce fever. Aspirin, or products containing aspirin, should not be given to children with chickenpox, as it can cause Reye's Syndrome.

In adults, the disease is more severe, though the incidence is much less common. Infection in adults is associated with greater morbidity and mortality due to pneumonia (either direct viral pneumonia or secondary bacterial pneumonia), bronchitis (either viral bronchitis or secondary bacterial bronchitis), hepatitis, and encephalitis. In particular, up to 10% of pregnant women with chickenpox develop pneumonia, the severity of which increases with onset later in gestation. In England and Wales, 75% of deaths due to chickenpox are in adults. Inflammation of the brain, or encephalitis, can occur in immunocompromised individuals, although the risk is higher with herpes zoster. Necrotizing fasciitis is also a rare complication.

Varicella can be lethal to adults with impaired immunity. The number of people in this high-risk group has increased, due to the HIV epidemic and the increased use of immunosuppressive therapies. Varicella is a particular problem in hospitals, when there are patients with immune systems weakened by drugs (e.g., high-dose steroids) or HIV.

Secondary bacterial infection of skin lesions, manifesting as impetigo, cellulitis, and erysipelas, is the most common complication in healthy children. Disseminated primary varicella infection usually seen in the immunocompromised may have high morbidity. Ninety percent of cases of varicella pneumonia occur in the adult population. Rarer complications of disseminated chickenpox include myocarditis, hepatitis, and glomerulonephritis.

Hemorrhagic complications are more common in the immunocompromised or immunosuppressed populations, although healthy children and adults have been affected. Five major clinical syndromes have been described: febrile purpura, malignant chickenpox with purpura, postinfectious purpura, purpura fulminans, and anaphylactoid purpura. These syndromes have variable courses, with febrile purpura being the most benign of the syndromes and having an uncomplicated outcome. In contrast, malignant chickenpox with purpura is a grave clinical condition that has a mortality rate of greater than 70%. The cause of these hemorrhagic chickenpox syndromes is not known.

Primary varicella occurs in all countries worldwide. In 2013 the disease resulted in 7,000 deaths – down from 8,900 in 1990.

In temperate countries, chickenpox is primarily a disease of children, with most cases occurring during the winter and spring, most likely due to school contact. It is one of the classic diseases of childhood, with the highest prevalence in the 4–10-year-old age group. Like rubella, it is uncommon in preschool children. Varicella is highly communicable, with an infection rate of 90% in close contacts. In temperate countries, most people become infected before adulthood, and 10% of young adults remain susceptible.

In the tropics, chickenpox often occurs in older people and may cause more serious disease. In adults, the pock marks are darker and the scars more prominent than in children.

In the United States, the Centers for Disease Control and Prevention (CDC) does not require state health departments to report infections of chickenpox, and only 31 states currently volunteer this information. However, in a 2013 study conducted by the social media disease surveillance tool called Sickweather, anecdotal reports of chickenpox infections on Facebook and Twitter were used to measure and rank states with the most infections per capita, with Maryland, Tennessee and Illinois in the top three.

Aleutian disease, also known as mink plasmacytosis, is a disease which causes spontaneous abortion and death in minks and ferrets. It is caused by "Carnivore amdoparvovirus 1" (also known as "Aleution diease virus", ADV), a highly contagious parvovirus in the genus "Amdoparvovirus".

The virus has been found as a natural infection in the "Mustelidae" family within mink, ferrets, otters, polecats, stone and pine martens and within other varying carnivores such as skunks, genets, foxes and raccoons. This is most commonly explained as because they all share resources and habitats.

Caseous lymphadenitis (CLA) is an infectious disease caused by the bacterium "Corynebacterium pseudotuberculosis" found mostly in goats and sheep that at present has no cure. It manifests itself predominantly in the form of large, pus-filled cysts on the neck, sides and udders of goats and sheep. The disease is spread mostly from an animal coming in contact with pus from a burst cyst on an infected animal, but the disease is highly contagious and is thought to also be spread by coughing or even by flies. Studies have found CL incidence in commercial goat herds as high as 30%.

Contagious equine metritis (CEM) is a type of metritis (uterine inflammation) in horses that is caused by a sexually transmitted infection. It is thus an equine venereal disease of the genital tract of horses, brought on by the "Taylorella equigenitalis" bacteria and spread through sexual contact. The disease was first reported in 1977, and has since been reported worldwide.

In the United States there are typically between a couple of hundred and a couple of thousand cases a year.

The aetiological agent of turkey viral hepatitis is a virus from the Picornaviridae family.

The disease is restricted to turkeys and is highly contagious but usually subclinical. It usually present in young birds under the age of 6 weeks.

It has been seen in Canada, Italy, the US and the UK.

Transmission is thought to be via the faeces and vertical transmission may also occur.

There is no specific treatment for the condition.

Control may rely on boosting bird immunity, preventing group mixing and faecal spreading.