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.

In some renal transplant patients, the necessary use of immunosuppressive drugs has the side-effect of allowing the virus to replicate within the graft, a disease called BK nephropathy.

From 1–10% of renal transplant patients progress to BK virus associated nephropathy (BKVAN) and up to 80% of these patients lose their grafts. The onset of nephritis can occur as early as several days post-transplant to as late as 5 years.

It is also associated with ureteral stenosis and interstitial nephritis. In bone marrow transplant recipients it is notable as a cause for hemorrhagic cystitis.

Characteristics of a viral infection can include pain, swelling, redness, impaired function, fever, drowsiness, confusion and convulsions.

The most common diseases caused by chronic viral infections are subacute-sclerosing panencephalitis, progressive multifocal leukoencephalopathy, retrovirus disease and spongiform encephalopathies.

Roseola typically affects children between six months and two years of age, and begins with a sudden high fever (39–40 °C; 102.2-104 °F). In rare cases, this can cause cause febrile convulsions (also known as febrile seizures or "fever fits") due to the sudden rise in body temperature, but in many cases the child appears normal. After a few days the fever subsides, and just as the child appears to be recovering, a red rash appears. This usually begins on the trunk and then spreads to the arms, legs, and neck. The rash is not itchy and may last 1 to 2 days. In contrast, a child suffering from measles would usually appear sicker, with symptoms of conjunctivitis, cold-like symptoms, and a cough, and their rash would affect the face and last for several days. Liver dysfunction can occur in rare cases.

A small percentage of children acquire HHV-6 with few sign or symptoms of the disease. Exanthema subitum occurs in approximately 30% of children during primary HHV-6 infection. Others may show symptoms significant enough that other more serious infections, such as meningitis or measles should be ruled out. In case of febrile seizures, medical advice can be sought for reassurance. However, febrile seizures are not harmful, do not require treatment, and have no long term negative effects unless they last longer than five minutes.

In rare cases, HHV-6 can become active in an adult previously infected during childhood and can show signs of mononucleosis.

An oncovirus is a virus that can cause cancer. This term originated from studies of acutely transforming retroviruses in the 1950–60s, often called oncornaviruses to denote their RNA virus origin.

It now refers to any virus with a DNA or RNA genome causing cancer and is synonymous with "tumor virus" or "cancer virus". The vast majority of human and animal viruses do not cause cancer, probably because of longstanding co-evolution between the virus and its host. Oncoviruses have been important not only in epidemiology, but also in investigations of cell cycle control mechanisms such as the Retinoblastoma protein.

The World Health Organization's International Agency for Research on Cancer estimated that in 2002, infection caused 17.8% of human cancers, with 11.9% caused by one of seven viruses. These cancers might be easily prevented through vaccination (e.g., papillomavirus vaccines), diagnosed with simple blood tests, and treated with less-toxic antiviral compounds.

Human parainfluenza viruses (HPIVs) are the viruses that cause human parainfluenza. HPIVs are a group of four distinct single-stranded RNA viruses belonging to the Paramyxoviridae family. These viruses are closely associated with both human and veterinary disease. Virions are approximately 150–250 nm in size and contain negative sense RNA with a genome encompassing ~15,000 nucleotides.

The viruses can be detected via cell culture, immunofluorescent microscopy, and PCR. HPIVs remain the second main cause of hospitalisation in children under 5 years of age suffering from a respiratory illness (only respiratory syncytial virus causes more respiratory hospitalisations for this age group).

Roseola is an infectious disease caused by certain types of virus. Most infections occur before the age of three. Symptoms vary from absent to the classic presentation of a fever of rapid onset followed by a rash. The fever generally lasts for three to five days. The rash is generally pink and lasts for less than three days. Complications may include febrile seizures, with serious complications being rare.

It is caused by either "human herpesvirus 6" (HHV-6) or "human herpesvirus 7" (HHV-7). Spread is usually through the saliva of those who are otherwise healthy. However, it may also spread from the mother to baby during pregnancy. Diagnosis is typically based on symptoms but can be confirmed with blood tests. Low numbers of white blood cells may also be present.

Treatment includes sufficient fluids and medications to treat the fever. Nearly all people are infected at some point in time. Males and females are affected equally often. The disease was first described in 1910 while the causal virus was determined in 1988. The disease may reactivate in those with a weakened immune system and may result in significant health problems.

The first HPIV was discovered in the late 1950s. The taxonomic division is broadly based on antigenic and genetic characteristics, forming four major serotypes or clades, which today are considered distinct viruses. These include:

HPIVs belong to two genera: "Respirovirus" (HPIV-1 & HPIV-3) and "Rubulavirus" (HPIV-2 & HPIV-4).

The disease is characterized by flesh-colored to erythematous (reddened) papules occurring in the central region of the face and sometimes elsewhere on the body, often accompanied by protrusive "spicules" or spines made of keratin and by alopecia of affected skin, typically the eyebrows and sometimes the eyelashes or scalp hairs. Pruritus (itching) has been described in about a third of reported cases. Facial papules are generally 1- to 3-mm in size. The condition is considered to be benign, but can be disfiguring; the spines are often prominent, and in later stages the affected facial skin thickens noticeably.

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.

Trichodysplasia spinulosa (also known by many other names, including viral-associated trichodysplasia spinulosa, viral-associated trichodysplasia, pilomatrix dysplasia and ciclosporin-induced folliculodystrophy, although the last is a misnomer) is a rare cutaneous condition that has been described almost exclusively in immunocompromised patients, usually organ transplant recipients, on regimens of immunosuppressive drugs. As of early 2016, a total of 32 cases had been reported in the medical literature. Despite its rarity, TS is believed to be underdiagnosed, and the growing population of patients on immunosuppressive drug regimens suggests its incidence may rise. TS has been described as an emerging infectious disease.

Borna disease is an infectious neurological syndrome of warm-blooded animals, caused by Borna disease viruses 1 and 2 (BoDV-1/2), both of which are members of the species "Mammalian 1 bornavirus". BoDV-1 an 2 cause abnormal behaviour and fatality. Borna disease viruses 1 and 2 are neurotropic viruses and members of the "Bornaviridae" family within the "Mononegavirales" order.

Although Borna disease viruses 1 and 2 are mainly seen as the causative agent of Borna disease in horses and other animals, they are also controversially discussed as human infectious agents and therefore as potential zoonotic agents. The role of BoDV-1 and -2 in human illness is controversial and it is yet to be established whether BoDV-1 or -2 cause any overt disease in humans. However, correlative evidence exists linking BoDV-1/2 infection with neuropsychiatric disorders such as bipolar disorder.

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.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the virus strain that causes coronavirus disease 2019 (COVID-19), a respiratory illness. It is colloquially known as the coronavirus, and was previously referred to by its provisional name 2019 novel coronavirus (2019-nCoV). SARS-CoV-2 is a positive-sense single-stranded RNA virus. It is contagious in humans, and the World Health Organization (WHO) has designated the ongoing pandemic of COVID-19 a Public Health Emergency of International Concern. Because the strain was first discovered in Wuhan, China, it is sometimes referred to as "Wuhan virus" or "Wuhan coronavirus". Since the WHO discourages the use of names based on locations such as MERS, and to avoid confusion with the disease SARS, it sometimes refers to SARS-CoV-2 as "the COVID-19 virus" in public health communications. The general public frequently calls both SARS-CoV-2 and the disease it causes "coronavirus", but scientists typically use more precise terminology.

Taxonomically, SARS-CoV-2 is a strain of Severe acute respiratory syndrome-related coronavirus (SARSr-CoV). It is believed to have zoonotic origins and has close genetic similarity to bat coronaviruses, suggesting it emerged from a bat-borne virus. An intermediate animal reservoir such as a pangolin is also thought to be involved in its introduction to humans. The virus shows little genetic diversity, indicating that the spillover event introducing SARS-CoV-2 to humans is likely to have occurred in late 2019.

Epidemiological studies estimate each infection results in 1.4 to 3.9 new ones when no members of the community are immune and no preventive measures taken. The virus is primarily spread between people through close contact and via respiratory droplets produced from coughs or sneezes. It mainly enters human cells by binding to the receptor angiotensin converting enzyme 2 (ACE2).

The mode of transmission of BoDV-1/2 is unclear but probably occurs through intranasal exposure to contaminated saliva or nasal secretions. Following infection, individuals may develop Borna disease, or may remain subclinical, possibly acting as a carrier of the virus.

Monkeypox is an infectious disease caused by the monkeypox virus. Symptoms begin with fever, headache, muscle pains, swollen lymph nodes, and feeling tired. This is then followed by a rash that forms blisters and crusts over. The time from exposure to onset of symptoms is around 10 days. The duration of symptoms is typically 2 to 5 weeks.

Monkeypox may be spread from handling bush meat, an animal bite or scratch, body fluids, contaminated objects, or close contact with an infected person. The virus is believed to normally circulate among certain rodents in Africa. Diagnosis can be confirmed by testing a lesion for the viruses DNA. The disease can appear similar to chickenpox.

The smallpox vaccine is believed to prevent infection. Cidofovir may be useful as treatment. The risk of death in those infected is up to 10%.

The disease mostly occurs in Central and West Africa. It was first identified in 1958 among laboratory monkeys. The first cases in humans were found in 1970 in the Democratic Republic of the Congo. An outbreak that occurred in the United States in 2003 was traced to a pet store where imported Gambian rodents were sold.

Generally, tumor viruses cause little or no disease after infection in their hosts, or cause non-neoplastic diseases such as acute hepatitis for hepatitis B virus or mononucleosis for Epstein-Barr virus. A minority of persons (or animals) will go on to develop cancers after infection. This has complicated efforts to determine whether or not a given virus causes cancer. The well-known Koch's postulates, 19th-century constructs developed by Robert Koch to establish the likelihood that "Bacillus anthracis" will cause anthrax disease, are not applicable to viral diseases. (Firstly, this is because viruses cannot truly be isolated in pure culture—even stringent isolation techniques cannot exclude undetected contaminating viruses with similar density characteristics, and viruses must be grown on cells. Secondly, asymptomatic virus infection and carriage is the norm for most tumor viruses, which violates Koch's third principle. Relman and Fredericks have described the difficulties in applying Koch's postulates to virus-induced cancers. Finally, the host restriction for human viruses makes it unethical to experimentally transmit a suspected cancer virus.) Other measures, such as A.B. Hill's criteria, are more relevant to cancer virology but also have some limitations in determining causality.

Tumor viruses come in a variety of forms: Viruses with a DNA genome, such as adenovirus, and viruses with an RNA genome, like the Hepatitis C virus (HCV), can cause cancers, as can retroviruses having both DNA and RNA genomes (Human T-lymphotropic virus and hepatitis B virus, which normally replicates as a mixed double and single-stranded DNA virus but also has a retroviral replication component). In many cases, tumor viruses do not cause cancer in their native hosts but only in dead-end species. For example, adenoviruses do not cause cancer in humans but are instead responsible for colds, conjunctivitis and other acute illnesses. They only become tumorigenic when infected into certain rodent species, such as Syrian hamsters. Some viruses are tumorigenic when they infect a cell and persist as circular episomes or plasmids, replicating separately from host cell DNA (Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus). Other viruses are only carcinogenic when they integrate into the host cell genome as part of a biological accident, such as polyomaviruses and papillomaviruses.

A direct oncogenic viral mechanism involves either insertion of additional viral oncogenic genes into the host cell or to enhance already existing oncogenic genes (proto-oncogenes) in the genome. Indirect viral oncogenicity involves chronic nonspecific inflammation occurring over decades of infection, as is the case for HCV-induced liver cancer. These two mechanisms differ in their biology and epidemiology: direct tumor viruses must have at least one virus copy in every tumor cell expressing at least one protein or RNA that is causing the cell to become cancerous. Because foreign virus antigens are expressed in these tumors, persons who are immunosuppressed such as AIDS or transplant patients are at higher risk for these types of cancers. Chronic indirect tumor viruses, on the other hand, can be lost (at least theoretically) from a mature tumor that has accumulated sufficient mutations and growth conditions (hyperplasia) from the chronic inflammation of viral infection. In this latter case, it is controversial but at least theoretically possible that an indirect tumor virus could undergo "hit-and-run" and so the virus would be lost from the clinically diagnosed tumor. In practical terms, this is an uncommon occurrence if it does occur.

The disease has a long incubation period, and therefore signs usually occur in adult animals (over 2 years of age). Clinical signs resemble a non-specific progressive pneumonia, including poor body condition and, particularly after exercise, respiratory difficulty. Unless a concurrent lung infection is present, affected sheep continue to eat. The only sign specific to OPA is a watery nasal discharge, consisting of lung fluid produced by the affected lung tissue; lifting the hind legs of the animal above the level of its head will cause large volumes of this fluid to flow from the nostrils.

There are no reliable tests for the diagnosis of OPA in live animals which are suitable for use on farms, so diagnosis can only be confirmed at necropsy (post-mortem examination). On necropsy, lungs are interspersed with multifocal tumors. Some of these are small discrete nodules and others will involve the entire half of a lung lobule. JSRV acutely transforms the lung epithelia into cancerous cells, with type-2 pneumocytes and club cells being the likely target for JSRV transformation. The tumors have overactive secretory functions, which are a hallmark of OPA.

The retroviral antigen levels of JSRV are very high in OPA tumors and can be detected in the lung secretions of infected sheep. It is thought that infected animals secrete the virus before showing signs, so the virus is easily spread within flocks.

Ovine pulmonary adenocarcinoma (OPA), also known as ovine pulmonary adenomatosis, or jaagsiekte, is a chronic and contagious disease of the lungs of sheep and goats. OPA is caused by a retrovirus called jaagsiekte sheep retrovirus (JSRV).

Monkeypox is similar to smallpox, although it is often milder.

Limited person-to-person spread of infection has been reported in disease-endemic areas in Africa. Case-fatality ratios in Africa have ranged from 1% to 10%.

Hepatitis E is a viral hepatitis (liver inflammation) caused by infection with a virus called hepatitis E virus. It is one of five known human hepatitis viruses: A, B, C, D, and E. HEV is a positive-sense, single-stranded, nonenveloped, RNA icosahedral virus; HEV has a fecal-oral transmission route. Infection with this virus was first documented in 1955 during an outbreak in New Delhi, India. A preventive vaccine (HEV 239) is approved for use in China.

Although hepatitis E often causes an acute and self-limiting infection (the virus usually resolves itself and the individual recovers) with low mortality rates in the western world, it bears a high risk of developing chronic hepatitis in immunocompromised patients with substantial mortality rates. Organ transplant recipients who receive immunosuppressive medication to prevent rejection are thought to be the main population at risk for chronic hepatitis E. Furthermore, in healthy individuals during the duration of the infection, the disease severely impairs a person’s ability to work, care for family members, and perform other daily activities. Hepatitis E occasionally develops into an acute, severe liver disease, and is fatal in about 2% of all cases. Clinically, it is comparable to hepatitis A, but in pregnant women, the disease is more often severe and is associated with a clinical syndrome called fulminant liver failure. Pregnant women, especially those in the third trimester, suffer an elevated mortality rate from the disease of around 20%.

Hepatitis E newly infected about 28 million people in 2013.

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

The incubation period of hepatitis E varies from 3 to 8 weeks. After a short prodromal phase symptoms lasting from days to weeks follow. They may include jaundice, fatigue, and nausea. The symptomatic phase coincides with elevated hepatic aminotransferase levels.

Viral RNA becomes detectable in stool and blood serum during incubation period. Serum IgM and IgG antibodies against HEV appear just before onset of clinical symptoms. Recovery leads to virus clearance from the blood, while the virus may persist in stool for much longer. Recovery is also marked by disappearance of IgM antibodies and increase of levels of IgG antibodies.

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.