Recent work has been done by virologists to learn more about the interference in infection of host cells and how DI genomes could potentially work as antiviral agents. The Dimmock & Easton, 2014 article explains that pre-clinical work is being done to test their effectiveness against influenza viruses. DI-RNAs have also been found to aid in the infection of fungi via viruses of the family "Partitiviridae" for the first time, which makes room for more interdisciplinary work.

Vaccines are available (ATCvet codes: for the inactivated vaccine, for the live vaccine, plus various combinations).

Given that avian reovirus infections are widespread, the viruses are relatively resistant outside the host, and that vertical and horizontal transmission occurs, eradicating avian reovirus infection in commercial chicken flocks is very unlikely. In addition, absence of detectable seroconversion and failure to detect virus in cloacal swabs are unreliable indicators of resisting infection, or transmission via the egg. Thus, the most proactive and successful approach to controlling this disease is through vaccination. Since chicks are more prone to being detrimentally affected by the disease right after hatching, vaccine protocols that use live and killed vaccines are designed to provide protection during the very early stages of life. This approach has been accomplished through active immunity after early vaccination and a live vaccine or passive immunity from maternal antibodies followed with vaccination of the breeder hens. Currently, efforts toward administering inactivated or live vaccines to breeding stock to allow passive immunity to the offspring via the yolk are being taken.

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.

Avian reoviruses belong to the genus "Orthoreovirus", and "Reoviridae" family. They are non-enveloped viruses that undergo replication in the cytoplasm of infected cells. It has icosahedral symmetry and contains a double-shelled arrangement of surface protein. Virus particles can range between 70–80 nm. Morphologically, the virus is a double stranded RNA virus that is composed of ten segments. The genome and proteins that are encoded by the genome can be separated into three different sizes ranging from small, medium, or large. Of the eleven proteins that are encoded for by the genome, two are nonstructural, while the remaining nine are structural.

Avian reoviruses can withstand a pH range of 3.0–9.0. Ambient temperatures are suitable for the survival of these viruses, which become inactive at 56 °C in less than an hour. Common areas where this virus can survive include galvanized metal, glass, rubber, feathers, and wood shavings. Avian reovirus can survive for up to ten days on these common areas in addition to up to ten weeks in water.

Cultivation and observation of the effects of avian reovirus is most often performed in chicken embryos. If infected into the yolk sac, the embryo will succumb to death accompanied by hemorrhaging of the embryos and cause the foci on the liver to appear yellowish-green. There are several primary chicken cell cultures/areas that are susceptible to avian reoviruses, which include the lungs, liver, kidney, and fibroblasts of the chick embryo. Of the following susceptible areas, liver cells from the chick embryo have been found to be the most sensitive for primary isolation from clinical material.

Typically, the CPE effect of avian reoviruses is the production of syncytia. CPE, or cytopathic effects are the visible changes in a host cell that takes place because of viral infection. Syncytia is a single cell or cytoplasmic mass containing several nuclei, formed by fusion of cells or by division of nuclei.

Viral entry is the earliest stage of infection in the viral life cycle, as the virus comes into contact with the host cell and introduces viral material into the cell. The major steps involved in viral entry are shown below. Despite the variation among viruses, there are several shared generalities concerning viral entry.

In virology, defective interfering particles (DIPs), also known as defective interfering viruses, are spontaneously generated virus mutants in which a critical portion of the particle's genome has been lost due to defective replication. DIPs are derived from and associated with their parent virus, and particles are classed as DIPs if they are rendered non-infectious due to at least one essential gene of the virus being lost or severely damaged as a result of the defection. A DIP can usually still penetrate host cells, but requires another fully functional virus particle (the 'helper' virus) to co-infect a cell with it, in order to provide the lost factors. The existence of DIPs has been known about for decades, and they can occur within nearly every class of both DNA and RNA viruses.

The theory that cancer could be caused by a virus began with the experiments of Oluf Bang and Vilhelm Ellerman in 1908 who first show that avian erythroblastosis (a form of chicken leukemia) could be transmitted by cell-free extracts. This was subsequently confirmed for solid tumors in chickens in 1910-1911 by Peyton Rous.

By the early 1950s it was known that viruses could remove and incorporate genes and genetic material in cells. It was suggested that these new genes inserted into cells could make the cell cancerous. Many of these viral oncogenes have been discovered and identified to cause cancer.

The main viruses associated with human cancers are human papillomavirus, hepatitis B and hepatitis C virus, Epstein-Barr virus, human T-lymphotropic virus, Kaposi's sarcoma-associated herpesvirus (KSHV) and Merkel cell polyomavirus. Experimental and epidemiological data imply a causative role for viruses and they appear to be the second most important risk factor for cancer development in humans, exceeded only by tobacco usage. The mode of virally induced tumors can be divided into two, "acutely transforming" or "slowly transforming". In acutely transforming viruses, the viral particles carry a gene that encodes for an overactive oncogene called viral-oncogene (v-onc), and the infected cell is transformed as soon as v-onc is expressed. In contrast, in slowly transforming viruses, the virus genome is inserted, especially as viral genome insertion is an obligatory part of retroviruses, near a proto-oncogene in the host genome. The viral promoter or other transcription regulation elements in turn cause overexpression of that proto-oncogene, which in turn induces uncontrolled cellular proliferation. Because viral genome insertion is not specific to proto-oncogenes and the chance of insertion near that proto-oncogene is low, slowly transforming viruses have very long tumor latency compared to acutely transforming viruses, which already carry the viral oncogene.

Hepatitis viruses, including hepatitis B and hepatitis C, can induce a chronic viral infection that leads to liver cancer in 0.47% of hepatitis B patients per year (especially in Asia, less so in North America), and in 1.4% of hepatitis C carriers per year. Liver cirrhosis, whether from chronic viral hepatitis infection or alcoholism, is associated with the development of liver cancer, and the combination of cirrhosis and viral hepatitis presents the highest risk of liver cancer development. Worldwide, liver cancer is one of the most common, and most deadly, cancers due to a huge burden of viral hepatitis transmission and disease.

Through advances in cancer research, vaccines designed to prevent cancer have been created. The hepatitis B vaccine is the first vaccine that has been established to prevent cancer (hepatocellular carcinoma) by preventing infection with the causative virus. In 2006, the U.S. Food and Drug Administration approved a human papilloma virus vaccine, called Gardasil. The vaccine protects against four HPV types, which together cause 70% of cervical cancers and 90% of genital warts. In March 2007, the US Centers for Disease Control and Prevention (CDC) Advisory Committee on Immunization Practices (ACIP) officially recommended that females aged 11–12 receive the vaccine, and indicated that females as young as age 9 and as old as age 26 are also candidates for immunization.

Prophylaxis by vaccination, as well as preventive measures like protective clothing, tick control, and mosquito control are advised. The vaccine for KFDV consists of formalin-inactivated KFDV. The vaccine has a 62.4% effectiveness rate for individuals who receive two doses. For individuals who receive an additional dose, the effectiveness increases to 82.9%. Specific treatments are not available.

The serious complications of HiB are brain damage, hearing loss, and even death.

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.

Effective vaccines for "Haemophilus influenzae" Type B have been available since the early 1990s, and is recommended for children under age 5 and asplenic patients. The World Health Organization recommends a pentavalent vaccine, combining vaccines against diphtheria, tetanus, pertussis, hepatitis B and Hib. There is not yet sufficient evidence on how effective this pentavalent vaccine is in relation to the individual vaccines.

Hib vaccines cost about seven times the total cost of vaccines against measles, polio, tuberculosis, diphtheria, tetanus, and pertussis. Consequently, whereas 92% of the populations of developed countries was vaccinated against Hib as of 2003, vaccination coverage was 42% for developing countries, and only 8% for least-developed countries.

The Hib vaccines do not provide cross-protection to any other "Haemophilus influenzae" serotypes like Hia, Hic, Hid, Hie or Hif.

There is no treatment currently available. The virus generally resolves itself within a five to seven day period. The use of steroids can actually cause a corneal microbial superinfection which then requires antimicrobial therapy to eliminate.

Rotavirus is highly contagious and cannot be treated with antibiotics or other drugs. Because improved sanitation does not decrease the prevalence of rotaviral disease, and the rate of hospitalisations remains high despite the use of oral rehydrating medicines, the primary public health intervention is vaccination. In 1998, a rotavirus vaccine was licensed for use in the United States. Clinical trials in the United States, Finland, and Venezuela had found it to be 80 to 100% effective at preventing severe diarrhoea caused by rotavirus A, and researchers had detected no statistically significant serious adverse effects. The manufacturer, however, withdrew it from the market in 1999, after it was discovered that the vaccine may have contributed to an increased risk for intussusception, a type of bowel obstruction, in one of every 12,000 vaccinated infants. The experience provoked intense debate about the relative risks and benefits of a rotavirus vaccine.

In 2006, two new vaccines against infection were shown to be safe and effective in children, and in 2009, the WHO recommended that rotavirus vaccine be included in all national immunisation programmes.

The incidence and severity of rotavirus infections has declined significantly in countries that have acted on this recommendation. A 2014 review of available clinical trial data from countries routinely using rotavirus vaccines in their national immunisation programs found that rotavirus vaccines have reduced rotavirus hospitalisations by 49–92 percent and all cause diarrhoea hospitalisations by 17–55 percent. In Mexico, which in 2006 was among the first countries in the world to introduce rotavirus vaccine, diarrhoeal disease death rates dropped during the 2009 rotavirus season by more than 65 percent among children age two and under. In Nicaragua, which in 2006 became the first developing country to introduce a rotavirus vaccine, severe rotavirus infections were reduced by 40 percent and emergency room visits by a half. In the United States, rotavirus vaccination since 2006 has led to drops in rotavirus-related hospitalisations by as much as 86 percent. The vaccines may also have prevented illness in non-vaccinated children by limiting the number of circulating infections. In developing countries in Africa and Asia, where the majority of rotavirus deaths occur, a large number of safety and efficacy trials as well as recent post-introduction impact and effectiveness studies of Rotarix and RotaTeq have found that vaccines dramatically reduced severe disease among infants. In September 2013, the vaccine was offered to all children in the UK, aged between two and three months, and it is expected to halve the cases of severe infection and reduce the number of children admitted to hospital because of the infection by 70 percent. In Europe, hospitalisation rates following infection by rotavirus have decreased by 65% to 84% following the introduction of the vaccine. Globally, vaccination has reduced hospital admissions and emergency department visits by a median of 67%.

Rotavirus vaccines are licensed in over 100 countries, and more than 80 countries have introduced routine rotavirus vaccination, almost half with the support of Gavi, the Vaccine Alliance. To make rotavirus vaccines available, accessible, and affordable in all countries—particularly low- and middle-income countries in Africa and Asia where the majority of rotavirus deaths occur, PATH (formerly Program for Appropriate Technology in Health), the WHO, the U.S. Centers for Disease Control and Prevention, and Gavi have partnered with research institutions and governments to generate and disseminate evidence, lower prices, and accelerate introduction.

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.

Lesions of paravaccinia virus will clear up with little to no scaring after 4 to 8 weeks. An antibiotic may be prescribed by a physician to help prevent bacterial infection of the lesion area. In rare cases, surgical removal of the lesions can be done to help increase rate of healing, and help minimize risk of bacterial or fungal infection. Upon healing, no long term side effects have been reported.

Rotaviruses infect the young of many species of animals and they are a major cause of diarrhoea in wild and reared animals worldwide. As a pathogen of livestock, notably in young calves and piglets, rotaviruses cause economic loss to farmers because of costs of treatment associated with high morbidity and mortality rates. These rotaviruses are a potential reservoir for genetic exchange with human rotaviruses. There is evidence that animal rotaviruses can infect humans, either by direct transmission of the virus or by contributing one or several RNA segments to reassortants with human strains.

The vaccine for hepatitis B protects against hepatitis D virus because of the latter's dependence on the presence of hepatitis B virus for it to replicate.

Latest evidence suggests that Pegylated interferon alpha is effective in reducing the viral load and the effect of the disease during the time the drug is given, but the benefit generally stops if the drug is discontinued. The efficiency of the pegylated interferon treatment does not usually exceed ~20%.

The drug myrcludex B, which inhibits virus entry into hepatocytes, is in clinical trials .

A vaccine has been conditionally approved for use in animals in the US. It has been shown that knockout of the NSs and NSm nonstructural proteins of this virus produces an effective vaccine in sheep as well.

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.

Exposure to antiretroviral treatments has led to the evolution of HIV in response to selection pressure that eliminates strains of HIV that do not express resistance mechanisms. Drug resistance occurs in all antiretroviral treatments if patients are non-adherent, meaning that they do not take their medication regimens as prescribed. Lack of adherence may result from unreliable access to the medication, due to prohibitive cost or inadequate supply.

Current medical and scientific opinion is mixed on the most effective treatment methods, but is focused on drug cocktails and the importance of first-line regimens . The World Health Organization advocates a public-health approach to HIV treatment in order to make treatment uniform and available to patients around the world. As of July 2017, the WHO is implementing the Global Action Plan on HIV drug resistance 2017-2021. It is a 5-year initiative intended to help countries around the world manage HIV drug resistance.

Among treatment methods, the World Health Organization acknowledges the importance of successful first-line treatments. First-line treatments are known to affect the virus’ future response to other treatments, making the effectiveness of first-line treatments an issue of vital importance. The most successful treatments are combinations of three drugs used simultaneously, as this greatly reduces the probability of the virus developing resistance.

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.

Based on the low variability exhibited among known SARS-CoV-2 genomic sequences, the strain is thought to have been detected by health authorities within weeks of its emergence among the human population in late 2019. The earliest case of infection currently known is thought to have been found on 17 November 2019. The virus subsequently spread to all provinces of China and to more than 150 other countries in Asia, Europe, North America, South America, Africa, and Oceania. Human-to-human transmission of the virus has been confirmed in all of these regions. On 30 January 2020, SARS-CoV-2 was designated a Public Health Emergency of International Concern by the WHO, and on 11 March 2020 the WHO declared it a pandemic.

The basic reproduction number (R0) of the virus has been estimated to be between 1.4 and 3.9. This means that each infection from the virus is expected to result in 1.4 to 3.9 new infections when no members of the community are immune and no preventive measures are taken. The reproduction number may be higher in densely populated conditions such as those found on cruise ships. Many forms of preventive efforts may be employed in specific circumstances in order to reduce the propagation of the virus.

There have been about 82,000 confirmed cases of infection in mainland China. While the proportion of infections that result in confirmed cases or progress to diagnosable disease remains unclear, one mathematical model estimated that on 25 January 2020 75,815 people were infected in Wuhan alone, at a time when the number of confirmed cases worldwide was only 2,015. Before 24 February 2020, over 95% of all deaths from COVID-19 worldwide had occurred in Hubei province, where Wuhan is located. As of 17 April 2020, the percentage had decreased to 2.1%.

As of 17 April 2020, there have been 2,234,109 total confirmed cases of SARS-CoV-2 infection in the ongoing pandemic. The total number of deaths attributed to the virus is 153,379. Many recoveries from confirmed infections go unreported, but at least 567,695 people have recovered from confirmed infections.

Previous methods of diagnosis included HI, complement fixation, neutralization tests, and injecting the serum of infected individuals into mice. However, new research has introduced more efficient methods to diagnose KFDV. These methods include: nested RT-PCR, TaqMan-based real-time RT-PCR, and immunoglobin M antibodies detection by ELISA. The two methods involving PCR are able to function by attaching a primer to the NS-5 gene which is highly conserved among the genus to which KFDV belongs. The last method allows for the detections of anti-KFDV antibodies in patients.

A superinfection is a second infection superimposed on an earlier one, especially by a different microbial agent of exogenous or endogenous origin, that is resistant to the treatment being used against the first infection. Examples of this in bacteriology are the overgrowth of endogenous "Clostridium difficile" which occurs following treatment with a broad-spectrum antibiotic, and pneumonia or septicemia from "Pseudomonas aeruginosa" in some immuno-compromised patients.

In virology, the definition is slightly different. Superinfection is the process by which a cell that has previously been infected by one virus gets co-infected with a different strain of the virus, or another virus, at a later point in time. Viral superinfections may be resistant to the antiviral drug or drugs that were being used to treat the original infection. Viral superinfections may also be less susceptible to the host's immune response.

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.