A recent study from The Cleveland Clinic reported that BK viremia load > 185 000 copies/ml at the time of first positive BKV diagnosis - to be the strongest predictor for BKVAN (97% specificity and 75% sensitivity). In addition the BKV peak viral loads in blood reaching 223 000 copies/ml at any time was found to be predictive for BKVAN (91% specificity and 88% sensitivity) .

It is not known how this virus is transmitted. It is known, however, that the virus is spread from person to person, and not from an animal source. It has been suggested that this virus may be transmitted through respiratory fluids or urine, since infected individuals periodically excrete virus in the urine. A survey of 400 healthy blood donors was reported as showing that 82% were positive for IgG against BK virus.

Numerous factors have been suggested and linked to a higher risk of acquiring the infection, inclusive of malnutrition, vitamin A deficiency, absence of breastfeeding during the early stages of life, environmental pollution and overcrowding.

Mortality caused by HPIVs in developed regions of the world remains rare. Where mortality has occurred, it is principally in the three core risk groups (very young, elderly and immuno-compromised). Long term changes can however be associated with airway remodelling and are believed to be a significant cause of morbidity. The exact associations between HPIVs and diseases such as chronic obstructive pulmonary disease (COPD) are still being investigated.

In developing regions of the world, the highest risk group in terms of mortality remains pre-school children. Mortality may be as a consequence of primary viral infection or secondary problems such as bacterial infection. Predispositions, such as malnutrition and other deficiencies may further elevate the chances of mortality associated with infection.

Overall, LRI's cause approximately 25–30% of total deaths in pre-school children in the developing world. HPIVs is believed to be associated with 10% of all LRI cases, thus remaining a significant cause of mortality.

Many viral infections of the central nervous system occur in seasonal peaks or as epidemics, whereas others, such as herpes simplex encephalitis, are sporadic. In endemic areas it is mostly a disease of children, but as the disease spreads to new regions, or nonimmune travelers visit endemic regions, nonimmune adults are also affected.

Meningitis is a very common in children. Newborns can develop herpes virus infections through contact with infected secretions in the birth canal. Other viral infections are acquired by breathing air contaminated with virus-containing droplets exhaled by an infected person. Arbovirus infections are acquired from bites by infected insects (called epidemic encephalitis). Viral central nervous system infections in newborns and infants usually begin with fever. The inability of infants to communicate directly makes it difficult to understand their symptoms. Newborns may have no other symptoms and may initially not otherwise appear ill. Infants older than a month or so typically become irritable and fussy and refuse to eat. Vomiting is common. Sometimes the soft spot on top of a newborn's head (fontanelle) bulges, indicating an increase in pressure on the brain. Because irritation of the meninges is worsened by movement, an infant with meningitis may cry more, rather than calm down, when picked up and rocked. Some infants develop a strange, high-pitched cry. Infants with encephalitis often have seizures or other abnormal movements. Infants with severe encephalitis may become lethargic and comatose and then die. To make the diagnosis of meningitis or the diagnosis of encephalitis, doctors do a spinal tap (lumbar puncture) to obtain cerebrospinal fluid (CSF) for laboratory analysis in children.

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.

Estimated percent of new cancers attributable to the virus worldwide in 2002. NA indicates not available.

The association of other viruses with human cancer is continually under research.

There is no specific vaccine against or treatment for exanthema subitum, and most children with the disease are not seriously ill.

Roseola is caused by two human herpesviruses, "human herpesvirus 6" (HHV-6) and "human herpesvirus 7" (HHV-7), which are sometimes referred to collectively as Roseolovirus. There are two variants of HHV-6 (HHV-6a and HHV-6b) and studies in the US, Europe, Dubai and Japan have shown that exanthema subitum is caused by HHV-6b. This form of HHV-6 infects over 90% of infants by age 2.

PML is most common in people with HIV1 infection; prior to the advent of effective antiretroviral therapy, as many as 5% of people with AIDS eventually developed PML. It is unclear why PML occurs more frequently in AIDS than in other immunosuppressive conditions; some research suggests the effects of HIV on brain tissue, or on JCV itself, make JCV more likely to become active in the brain and increase its damaging inflammatory effects.

PML can occur in people on chronic immunosuppressive therapy like corticosteroids, for organ transplant, in people with cancer (such as Hodgkin’s disease, leukemia, or lymphoma) and individuals with autoimmune diseases such as multiple sclerosis, rheumatoid arthritis, psoriasis, sarcoidosis, and systemic lupus erythematosus with or without biological therapies that depress the immune response and allow JC virus reactivation. These therapies include efalizumab, belatacept, rituximab, natalizumab, infliximab, cytotoxic chemotherapy, corticosteroids, and various transplant drugs such as tacrolimus.

The cause of PML is a type of polyomavirus called the JC virus (JCV), after the initials of the person from whose tissue the virus was first successfully cultured. Recent publications indicate 39% to 58% of the general population are seropositive for antibodies to JCV, indicating current or previous infection with the virus. Other publications put the percentage at 70% to 90% of the general population. JCV causes persistent asymptomatic infection in approximately one-third of the adult population, based on viral shedding into the urine from the site of asymptomatic infection in the kidney. The virus causes disease only when the immune system has been severely weakened.

TS has been reported almost exclusively in immunocompromised patients, primarily organ transplant recipients on regimens of immunosuppressive drugs, and also in patients with hematolymphoid malignancies. Interestingly, as of 2016 there were no case reports in the literature describing cases of TS in patients with HIV-AIDS.

There is compelling evidence that trichodysplasia spinulosa is caused by a polyomavirus called trichodysplasia spinulosa polyomavirus (TSPyV) or "Human polyomavirus 8". There is evidence that exposure to the virus is common among healthy adults; estimates of seroprevalence (that is, prevalence of detectable antibodies against viral proteins) in immunocompetent adults range from 70 to 80% in different sample populations. TSPyV infects the skin, but viral DNA is rarely detectable there in asymptomatic individuals even if they possess antibodies to the virus indicating exposure. It is not known whether TS represents new primary infection or opportunistic reactivation of a latent infection.

TS is considered to be a benign dysplasia, although it can be disfiguring and is sometimes itchy. It is not known whether TS lesions have the potential to develop into cancer; while this outcome has never been reported, some polyomaviruses are oncogenic. The natural history of untreated TS is not known and no long-term studies of its progress have been performed. Improvement in immune function has been reported to resolve symptoms in some individual cases. Treatment with antiviral drugs has also been reported to improve symptoms, but only as long as treatment continues.

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.

Currently, there is no proven, safe treatment for monkeypox. The people who have been infected can be vaccinated up to 14 days after exposure.

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.

Vaccination against smallpox is assumed to provide protection against human monkeypox infection considering they are closely related viruses and the vaccine protects animals from experimental lethal monkeypox challenge. This has not been conclusively demonstrated in humans because routine smallpox vaccination was discontinued following the apparent eradication of smallpox and due to safety concerns with the vaccine.

Smallpox vaccine has been reported to reduce the risk of monkeypox among previously vaccinated persons in Africa. The decrease in immunity to poxviruses in exposed populations is a factor in the prevalence of monkeypox. It is attributed both to waning cross-protective immunity among those vaccinated before 1980 when mass smallpox vaccinations were discontinued, and to the gradually increasing proportion of unvaccinated individuals. The United States Centers for Disease Control and Prevention (CDC) recommends that persons investigating monkeypox outbreaks and involved in caring for infected individuals or animals should receive a smallpox vaccination to protect against monkeypox. Persons who have had close or intimate contact with individuals or animals confirmed to have monkeypox should also be vaccinated.

CDC does not recommend preexposure vaccination for unexposed veterinarians, veterinary staff, or animal control officers, unless such persons are involved in field investigations.

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).

Human-to-human transmission of SARS-CoV-2 has been confirmed during the 2019–20 coronavirus pandemic. Transmission occurs primarily via respiratory droplets from coughs and sneezes within a range of about 1.8 metres (6 ft). Indirect contact via contaminated surfaces is another possible cause of infection. Preliminary research indicates that the virus may remain viable on plastic and steel for up to three days, but does not survive on cardboard for more than one day or on copper for more than four hours; the virus is inactivated by soap, which destabilises its lipid bilayer. Viral RNA has also been found in stool samples from infected individuals.

The degree to which the virus is infectious during the incubation period is uncertain, but research has indicated that the pharynx reaches peak viral load approximately four days after infection. On 1 February 2020, the World Health Organization (WHO) indicated that "transmission from asymptomatic cases is likely not a major driver of transmission". However, an epidemiological model of the beginning of the outbreak in China suggested that "pre-symptomatic shedding may be typical among documented infections" and that subclinical infections may have been the source of a majority of infections.

There is some evidence of human-to-animal transmission of SARS-CoV-2, including examples in felids. Some institutions have advised those infected with SARS-CoV-2 to restrict contact with animals.

The hepatitis E virus causes around 20 million infections a year. These result in around three million acute illnesses and as of 2010, 57,000 deaths annually. It is particularly dangerous for pregnant women, who can develop an acute form of the disease that is lethal in 30% of cases or more. HEV is a major cause of illness and of death in the developing world and disproportionate cause of deaths among pregnant women. Hepatitis E is endemic in Central Asia, while Central America and the Middle East have reported outbreaks.

Immunodeficiency or immunosuppression can be caused by:

- Malnutrition

- Fatigue

- Recurrent infections

- Immunosuppressing agents for organ transplant recipients

- Advanced HIV infection

- Chemotherapy for cancer

- Genetic predisposition

- Skin damage

- Antibiotic treatment leading to disruption of the physiological microbiome, thus allowing some microorganisms to outcompete others and become pathogenic (e.g. disruption of intestinal flora may lead to "Clostridium difficile" infection

- Medical procedures

- Pregnancy

- Ageing

- Leukopenia (i.e. neutropenia and lymphocytopenia)

The lack of or the disruption of normal vaginal flora allows the proliferation of opportunistic microorganisms and will cause the opportunistic infection - bacterial vaginosis.

Globally, symptomatic HAV infections are believed to occur in around 1.4 million people a year. About 114 million infections (asymptomatic and symptomatic) occurred all together in 2015. Acute hepatitis A resulted in 11,200 deaths in 2015. Developed countries have low circulating levels of hepatovirus A, while developing countries have higher levels of circulation. Most adolescents and adults in developing countries have already had the disease, thus are immune. Adults in midlevel countries may be at risk of disease with the potential of being exposed.

OPA has been found in most countries where sheep are farmed, with the exception of Australia and New Zealand. OPA has been eradicated in Iceland.

No breed or sex of sheep appears to be predisposed to OPA. Most affected sheep show signs at 2 to 4 years of age.

OPA is not a notifiable disease, and therefore it is difficult to assess its prevalence.

Since opportunistic infections can cause severe disease, much emphasis is placed on measures to prevent infection. Such a strategy usually includes restoration of the immune system as soon as possible, avoiding exposures to infectious agents, and using antimicrobial medications ("prophylactic medications") directed against specific infections.