The BK virus rarely causes disease but is typically associated with patients who have had a kidney transplant; many people who are infected with this virus are asymptomatic. If symptoms do appear, they tend to be mild: respiratory infection or fever. These are known as primary BK infections.

The virus then disseminates to the kidneys and urinary tract where it persists for the life of the individual. It is thought that up to 80% of the population contains a latent form of this virus, which remains latent until the body undergoes some form of immunosuppression. Typically, this is in the setting of kidney transplantation or multi-organ transplantation. Presentation in these immunocompromised individuals is much more severe. Clinical manifestations include renal dysfunction (seen by a progressive rise in serum creatinine), and an abnormal urinalysis revealing renal tubular cells and inflammatory cells.

The BK virus was first isolated in 1971 from the urine of a renal transplant patient, initials B.K. The BK virus is similar to another virus called the JC virus (JCV), since their genomes share 75% sequence similarity. Both of these viruses can be identified and differentiated from each other by carrying out serological tests using specific antibodies or by using a PCR based genotyping approach.

Pathogenesis occurs in the same manner in hamsters as in mice. Symptoms in hamsters are highly variable, and typically indicate that the pet has been infected and shedding the virus for several months. Early signs may include inactivity, loss of appetite, and a rough coat. As the disease progresses, the animal may experience weight loss, hunched posture, inflammation around the eyes, and eventually death. Alternatively, some infected hamsters may be asymptomatic.

LCMV causes callitrichid hepatitis in New World primates. The onset of the infection is nonspecific and may include fever, anorexia, dyspnea, weakness and lethargy. Jaundice is characteristic and petechial hemorrhages may develop. Prostration and death usually follow.

Necropsy lesions in primates with callitrichid hepatitis show signs of jaundice, hepatomegaly, splenomegaly, and subcutaneous and intramuscular hemorrhages. Pleural and pericardial effusion, sometimes sanguineous, has also been reported. On histology, multifocal necrosis with acidophilic bodies and mild inflammatory infiltrates are typically found in the liver.

Affected turkeys may show systemic signs such as anorexia, lethargy and death. Hepatic encephalopathy occurs secondary to liver involvement.

Egg hatching and production can be affected.

Diagnosis relies on isolation of the virus from samples of internal organs or the faeces inoculated into embryonated chicken eggs.

The main necropsy findings are multiple necrotic lesions on the liver and sometimes the spleen.

Early symptoms of hepatitis A infection can be mistaken for influenza, but some sufferers, especially children, exhibit no symptoms at all. Symptoms typically appear 2 to 6 weeks (the incubation period) after the initial infection. About 90% of children do not have symptoms. The time between infection and symptoms, in those who develop them, is between 2 and 6 weeks with an average of 28 days.

The risk for symptomatic infection is directly related to age, with more than 80% of adults having symptoms compatible with acute viral hepatitis and the majority of children having either asymptomatic or unrecognized infections.

Symptoms usually last less than 2 months, although some people can be ill for as long as 6 months:

- Fatigue

- Fever

- Nausea

- Appetite loss

- Jaundice, a yellowing of the skin or the whites of the eyes owing to hyperbilirubinemia

- Bile is removed from the bloodstream and excreted in the urine, giving it a dark amber colour

- Diarrhea

- Light, or clay-coloured faeces (acholic faeces)

- Abdominal discomfort

Bovine viral diarrhea (BVD) or bovine viral diarrhoea (UK English), and previously referred to as bovine virus diarrhoea (BVD), is a significant economic disease of cattle that is endemic in the majority of countries throughout the world. The causative agent, bovine viral diarrhea virus (BVDV), is a member of the "Pestivirus" genus of the family Flaviviridae.

BVD infection results in a wide variety of clinical signs, due to its immunosuppressive effects, as well as having a direct effect on respiratory disease and fertility. In addition, BVD infection of a susceptible dam during a certain period of gestation can result in the production of a persistently infected (PI) fetus.

PI animals recognise intra-cellular BVD viral particles as ‘self’ and shed virus in large quantities throughout life; they represent the cornerstone of the success of BVD as a disease.

An emerging infectious disease (EID) is an infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human pathogens. EIDs are caused by newly identified species or strains (e.g. Severe acute respiratory syndrome, HIV/AIDS) that may have evolved from a known infection (e.g. influenza) or spread to a new population (e.g. West Nile fever) or to an area undergoing ecologic transformation (e.g. Lyme disease), or be "reemerging" infections, like drug resistant tuberculosis. Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and non-infectious conditions leading to the development of novel syndemics. Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.

BVDV infection has a wide manifestation of clinical signs including fertility issues, milk drop, pyrexia, diarrhoea and fetal infection. Occasionally, a severe acute form of BVD may occur. These outbreaks are characterized by thrombocytopenia with high morbidity and mortality. However, clinical signs are frequently mild and infection insidious, recognised only by BVDV’s immunosuppressive effects perpetuating other circulating infectious diseases (particularly scours and pneumonias).

Hepatitis A is an infectious disease of the liver caused by the hepatitis A virus (HAV). Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop them, is between two and six weeks. When symptoms occur, they typically last eight weeks and may include nausea, vomiting, diarrhea, jaundice, fever, and abdominal pain. Around 10–15% of people experience a recurrence of symptoms during the six months after the initial infection. Acute liver failure may rarely occur, with this being more common in the elderly.

It is usually spread by eating food or drinking water contaminated with infected feces. Shellfish which have not been sufficiently cooked are a relatively common source. It may also be spread through close contact with an infectious person. While children often do not have symptoms when infected, they are still able to infect others. After a single infection, a person is immune for the rest of his or her life. Diagnosis requires blood testing, as the symptoms are similar to those of a number of other diseases. It is one of five known hepatitis viruses: A, B, C, D, and E.

The hepatitis A vaccine is effective for prevention. Some countries recommend it routinely for children and those at higher risk who have not previously been vaccinated. It appears to be effective for life. Other preventive measures include hand washing and properly cooking food. No specific treatment is available, with rest and medications for nausea or diarrhea recommended on an as-needed basis. Infections usually resolve completely and without ongoing liver disease. Treatment of acute liver failure, if it occurs, is with liver transplantation.

Globally, around 1.4 million symptomatic cases occur each year and about 114 million infections (symptomatic and asymptomatic). It is more common in regions of the world with poor sanitation and not enough safe water. In the developing world, about 90% of children have been infected by age 10, thus are immune by adulthood. It often occurs in outbreaks in moderately developed countries where children are not exposed when young and vaccination is not widespread. Acute hepatitis A resulted in 11,200 deaths in 2015. World Hepatitis Day occurs each year on July 28 to bring awareness to viral hepatitis.

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.

The most detailed study on the frequency, onset, and duration of MVD clinical signs and symptoms was performed during the 1998–2000 mixed MARV/RAVV disease outbreak. A maculopapular rash, petechiae, purpura, ecchymoses, and hematomas (especially around needle injection sites) are typical hemorrhagic manifestations. However, contrary to popular belief, hemorrhage does not lead to hypovolemia and is not the cause of death (total blood loss is minimal except during labor). Instead, death occurs due to multiple organ dysfunction syndrome (MODS) due to fluid redistribution, hypotension, disseminated intravascular coagulation, and focal tissue necroses.

Clinical phases of Marburg Hemorrhagic Fever's presentation are described below. Note that phases overlap due to variability between cases.

1. Incubation: 2–21 days, averaging 5–9 days.

2. Generalization Phase: Day 1 up to Day 5 from onset of clinical symptoms. MHF presents with a high fever (~40˚C) and a sudden, severe headache, with accompanying chills, fatigue, nausea, vomiting, diarrhea, pharyngitis, maculopapular rash, abdominal pain, conjunctivitis, & malaise.

3. Early Organ Phase: Day 5 up to Day 13. Symptoms include prostration, dyspnea, edema, conjunctival injection, viral exanthema, and CNS symptoms, including encephalitis, confusion, delirium, apathy, and aggression. Hemorrhagic symptoms typically occur late and herald the end of the early organ phase, leading either to eventual recovery or worsening & death. Symptoms include bloody stools, ecchymoses, blood leakage from venipuncture sites, mucosal & visceral hemorrhaging, and possibly hematemesis.

4. Late Organ Phase: Day 13 up to Day 21+. Symptoms bifurcate into two constellations for survivors & fatal cases. Survivors will enter a convalescence phase, experiencing myalgia, fibromyalgia, hepatitis, asthenia, ocular symptoms, & psychosis. Fatal cases continue to deteriorate, experiencing continued fever, obtundation, coma, convulsions, diffuse coagulopathy, metabolic disturbances, shock and death, with death typically occurring between Days 8 and 16.

infection causes acute symptoms in 15% of cases. Symptoms are generally mild and vague, including a decreased appetite, fatigue, nausea, muscle or joint pains, and weight loss and rarely does acute liver failure result. Most cases of acute infection are not associated with jaundice. The infection resolves spontaneously in 10–50% of cases, which occurs more frequently in individuals who are young and female.

Acute infection with virus is associated with acute viral hepatitis, an illness that begins with general ill-health, loss of appetite, nausea, vomiting, body aches, mild fever, and dark urine, and then progresses to development of jaundice. It has been noted that itchy skin has been an indication as a possible symptom of all hepatitis virus types. The illness lasts for a few weeks and then gradually improves in most affected people. A few people may have a more severe form of liver disease known as (fulminant hepatic failure) and may die as a result. The infection may be entirely asymptomatic and may go unrecognized.

Chronic infection with virus either may be asymptomatic or may be associated with a chronic inflammation of the liver (chronic hepatitis), leading to cirrhosis over a period of several years. This type of infection dramatically increases the incidence of hepatocellular carcinoma (HCC; liver cancer). Across Europe, hepatitis B and C cause approximately 50% of hepatocellular carcinomas. Chronic carriers are encouraged to avoid consuming alcohol as it increases their risk for cirrhosis and liver cancer. virus has been linked to the development of membranous glomerulonephritis (MGN).

Symptoms outside of the liver are present in 1–10% of HBV-infected people and include serum-sickness–like syndrome, acute necrotizing vasculitis (polyarteritis nodosa), membranous glomerulonephritis, and papular acrodermatitis of childhood (Gianotti–Crosti syndrome). The serum-sickness–like syndrome occurs in the setting of acute , often preceding the onset of jaundice. The clinical features are fever, skin rash, and polyarteritis. The symptoms often subside shortly after the onset of jaundice but can persist throughout the duration of acute . About 30–50% of people with acute necrotizing vasculitis (polyarteritis nodosa) are HBV carriers. HBV-associated nephropathy has been described in adults but is more common in children. Membranous glomerulonephritis is the most common form. Other immune-mediated hematological disorders, such as essential mixed cryoglobulinemia and aplastic anemia have been described as part of the extrahepatic manifestations of HBV infection, but their association is not as well-defined; therefore, they probably should not be considered etiologically linked to HBV.

There is a rapid onset of clinical signs over the period of 2–7 days, beginning with anorexia, lethargy, and hyperbilirubinemia (icterus and discolored urine). Signs of hepatic encephalopathy (ataxia, blindness, aggression, and coma) and fever can also occur. Other signs include photodermatitis, hemorrhagic diathesis, dependent edema, and colic. The reason for colic is unknown, but is thought to be due to rapid decrease in the size of the liver, and the increased risk of gastric impaction. Rarely, weight loss can occur.

A viral disease (or viral infection) occurs when an organism's body is invaded by pathogenic viruses, and infectious virus particles (virions)

attach to and enter susceptible cells.

Viral disease is usually detected by clinical presentation, for instance severe muscle and joint pains preceding fever, or skin rash and swollen lymph glands.

Laboratory investigation is not directly effective in detecting viral infections, because they do not themselves increase the white blood cell count. Laboratory investigation may be useful in diagnosing associated bacterial infections, however.

Viral infections are commonly of limited duration, so treatment usually consists in reducing the symptoms; antipyretic and analgesic drugs are commonly prescribed.

A subclinical infection (sometimes called a preinfection) is an infection that, being , is nearly or completely asymptomatic (no signs or symptoms). A subclinically infected person is thus an asymptomatic carrier of a microbe, intestinal parasite, or virus that usually is a pathogen causing illness, at least in some individuals. Many pathogens spread by being silently carried in this way by some of their host population. Such infections occur both in humans and nonhuman animals. An example of an asymptomatic infection is a mild common cold that is not noticed by the infected individual. Since subclinical infections often occur without eventual overt sign, their existence is only identified by microbiological culture or DNA techniques such as polymerase chain reaction.

Marburg virus is a hemorrhagic fever virus of the "Filoviridae" family of viruses and a member of the species "Marburg marburgvirus", genus "Marburgvirus". Marburg virus (MARV) causes Marburg virus disease in humans and nonhuman primates, a form of viral hemorrhagic fever. Considered to be extremely dangerous, the WHO rates it as a Risk Group 4 Pathogen (requiring biosafety level 4-equivalent containment). In the United States, the NIH/National Institute of Allergy and Infectious Diseases ranks it as a Category A Priority Pathogen and the Centers for Disease Control and Prevention lists it as a Category A Bioterrorism Agent. It is also listed as a biological agent for export control by the Australia Group.

The virus can be transmitted by exposure to one species of fruit bats or it can be transmitted between people via body fluids through unprotected copulation and broken skin. The disease can cause bleeding (haemorrhage), fever and other symptoms much like Ebola. Funeral rituals are a particular risk. Actual treatment of the virus after infection is not possible but early, professional treatment of symptoms like dehydration considerably increase survival chances.

In 2009, expanded clinical trials of an Ebola and Marburg vaccine began in Kampala, Uganda.

About 80% of those exposed to the virus develop a chronic infection. This is defined as the presence of detectable viral replication for at least six months. Most experience minimal or no symptoms during the initial few decades of the infection. Chronic can be associated with fatigue and mild cognitive problems. Chronic infection after several years may cause cirrhosis or liver cancer. The liver enzymes are normal in 7–53%. Late relapses after apparent cure have been reported, but these can be difficult to distinguish from reinfection.

Fatty changes to the liver occur in about half of those infected and are usually present before cirrhosis develops. Usually (80% of the time) this change affects less than a third of the liver. Worldwide hepatitis C is the cause of 27% of cirrhosis cases and 25% of hepatocellular carcinoma. About 10–30% of those infected develop cirrhosis over 30 years. Cirrhosis is more common in those also infected with hepatitis B, schistosoma, or HIV, in alcoholics and in those of male gender. In those with hepatitis C, excess alcohol increases the risk of developing cirrhosis 100-fold. Those who develop cirrhosis have a 20-fold greater risk of hepatocellular carcinoma. This transformation occurs at a rate of 1–3% per year. Being infected with hepatitis B in addition to hepatitis C increases this risk further.

Liver cirrhosis may lead to portal hypertension, ascites (accumulation of fluid in the abdomen), easy bruising or bleeding, varices (enlarged veins, especially in the stomach and esophagus), jaundice, and a syndrome of cognitive impairment known as hepatic encephalopathy. Ascites occurs at some stage in more than half of those who have a chronic infection.

Hepatitis B is an infectious disease caused by the hepatitis B virus (HBV) that affects the liver. It can cause both acute and chronic infections. Many people have no symptoms during the initial infection. Some develop a rapid onset of sickness with vomiting, yellowish skin, tiredness, dark urine and abdominal pain. Often these symptoms last a few weeks and rarely does the initial infection result in death. It may take 30 to 180 days for symptoms to begin. In those who get infected around the time of birth 90% develop chronic while less than 10% of those infected after the age of five do. Most of those with chronic disease have no symptoms; however, cirrhosis and liver cancer may eventually develop. These complications result in the death of 15 to 25% of those with chronic disease.

The virus is transmitted by exposure to infectious blood or body fluids. Infection around the time of birth or from contact with other people's blood during childhood is the most frequent method by which hepatitis B is acquired in areas where the disease is common. In areas where the disease is rare, intravenous drug use and sexual intercourse are the most frequent routes of infection. Other risk factors include working in healthcare, blood transfusions, dialysis, living with an infected person, travel in countries where the infection rate is high, and living in an institution. Tattooing and acupuncture led to a significant number of cases in the 1980s; however, this has become less common with improved sterility. The viruses cannot be spread by holding hands, sharing eating utensils, kissing, hugging, coughing, sneezing, or breastfeeding. The infection can be diagnosed 30 to 60 days after exposure. The diagnosis is usually confirmed by testing the blood for parts of the virus and for antibodies against the virus. It is one of five known hepatitis viruses: A, B, C, D, and E.

The infection has been preventable by vaccination since 1982. Vaccination is recommended by the World Health Organization in the first day of life if possible. Two or three more doses are required at a later time for full effect. This vaccine works about 95% of the time. About 180 countries gave the vaccine as part of national programs as of 2006. It is also recommended that all blood be tested for hepatitis B before transfusion and condoms be used to prevent infection. During an initial infection, care is based on the symptoms that a person has. In those who develop chronic disease, antiviral medication such as tenofovir or interferon may be useful; however, these drugs are expensive. Liver transplantation is sometimes used for cirrhosis.

About a third of the world population has been infected at one point in their lives, including 343 million who have chronic infections. Another 129 million new infections occurred in 2013. Over 750,000 people die of hepatitis B each year. About 300,000 of these are due to liver cancer. The disease is now only common in East Asia and sub-Saharan Africa where between 5 and 10% of adults are chronically infected. Rates in Europe and North America are less than 1%. It was originally known as "serum hepatitis". Research is looking to create foods that contain HBV vaccine. The disease may affect other great apes as well.

Marburg virus disease (MVD) is the official name listed in the World Health Organization's International Statistical Classification of Diseases and Related Health Problems 10 (ICD-10) for the human disease caused by any of the two marburgviruses Marburg virus (MARV) and Ravn virus (RAVV). In the scientific literature, Marburg hemorrhagic fever (MHF) is often used as an unofficial alternative name for the same disease. Both disease names are derived from the German city Marburg, where MARV was first discovered.

The strongest evidence linking EBV and cancer formation is found in Burkitt's lymphoma

and nasopharyngeal carcinoma. Additionally, it has been postulated to be a trigger for a subset of chronic fatigue syndrome patients as well as multiple sclerosis and other autoimmune diseases.

Burkitt's lymphoma is a type of Non-Hodgkin's lymphoma and is most common in equatorial Africa and is co-existent with the presence of malaria. Malaria infection causes reduced immune surveillance of B cells immortalized by EBV, resulting in an excessive number of B cells and an increased likelihood of an unchecked mutation. Repeated mutations can lead to loss of cell-cycle control, causing excessive proliferation observed as Burkitt's lymphoma. Burkitt's lymphoma commonly affects the jaw bone, forming a huge tumor mass. It responds quickly to chemotherapy treatment, namely cyclophosphamide, but recurrence is common.

Other B cell lymphomas arise in immunocompromised patients such as those with AIDS or who have undergone organ transplantation with associated immunosuppression (Post-Transplant Lymphoproliferative Disorder (PTLPD)). Smooth muscle tumors are also associated with the virus in malignant patients.

Nasopharyngeal carcinoma is a cancer found in the upper respiratory tract, most commonly in the nasopharynx, and is linked to the EBV virus. It is found predominantly in Southern China and Africa, due to both genetic and environmental factors. It is much more common in people of Chinese ancestry (genetic), but is also linked to the Chinese diet of a high amount of smoked fish, which contain nitrosamines, well known carcinogens (environmental).

In 80% of cases, the disease is asymptomatic, but in the remaining 20%, it takes a complicated course. The virus is estimated to be responsible for about 5,000 deaths annually. The fever accounts for up to one-third of deaths in hospitals within the affected regions and 10 to 16% of total cases.

After an incubation period of six to 21 days, an acute illness with multiorgan involvement develops. Nonspecific symptoms include fever, facial swelling, and muscle fatigue, as well as conjunctivitis and mucosal bleeding. The other symptoms arising from the affected organs are:

- Gastrointestinal tract

- Nausea

- Vomiting (bloody)

- Diarrhea (bloody)

- Stomach ache

- Constipation

- Dysphagia (difficulty swallowing)

- Hepatitis

- Cardiovascular system

- Pericarditis

- Hypertension

- Hypotension

- Tachycardia (abnormally high heart rate)

- Respiratory tract

- Cough

- Chest pain

- Dyspnoea

- Pharyngitis

- Pleuritis

- Nervous system

- Encephalitis

- Meningitis

- Unilateral or bilateral hearing deficit

- Seizures

Clinically, Lassa fever infections are difficult to distinguish from other viral hemorrhagic fevers such as Ebola and Marburg, and from more common febrile illnesses such as malaria.

The virus is excreted in urine for 3–9 weeks and in semen for three months.

Epstein–Barr can cause infectious mononucleosis, also known as 'glandular fever', 'mono' and 'Pfeiffer's disease'. Infectious mononucleosis is caused when a person is first exposed to the virus during or after adolescence. It is predominantly found in the developing world, and most children in the developing world are found to have already been infected by around 18 months of age. Infection of children can occur when adults mouth feed or pre-chew food before giving it to the child. EBV antibody tests turn up almost universally positive. In the United States roughly half of five-year-olds have been infected.