The incubation period is 5–7 days (with a range of 3–10). Symptoms can include a harsh, dry cough, retching, sneezing, snorting, gagging or vomiting in response to light pressing of the trachea or after excitement or exercise. The presence of a fever varies from case to case.

Initial symptoms are flu-like and may include fever, myalgia, lethargy symptoms, cough, sore throat, and other nonspecific symptoms. The only symptom common to all patients appears to be a fever above . SARS may eventually lead to shortness of breath and/or pneumonia; either direct viral pneumonia or first bacterial pneumonia.

Coughing and rattling are common, most severe in young, such as broilers, and rapidly spreading in chickens confined or at proximity. Morbidity is 100% in non-vaccinated flocks. Mortality varies according to the virus strain (up to 60% in non-vaccinated flocks). Respiratory signs will subdue within two weeks. However, for some strains, a kidney infection may follow, causing mortality by toxemia. Younger chickens may die of tracheal occlusion by mucus (lower end) or by kidney failure. The infection may prolong in the cecal tonsils.

In laying hens, there can be transient respiratory signs, but mortality may be negligible. However, egg production drops sharply. A great percentage of produced eggs are misshapen and discolored. Many laid eggs have a thin or soft shell and poor albumen (watery), and are not marketable or proper for incubation. Normally-colored eggs, indicative of normal shells for instance in brown chickens, have a normal hatchability.

Egg yield curve may never return to normal. Milder strains may allow normal production after around eight weeks.

Although kennel cough is considered to be a multifactorial infection, there are two main forms. The first is more mild and is caused by B. bronchiseptica and canine parainfluenza virus infections, without complications from canine distemper virus (CDV) or canine adenovirus (CAV). This form occurs most regularly in autumn, and can be distinguished by symptoms such as a retching cough and vomiting. The second form has a more complex combination of causative organisms including CDV and CAV. It typically occurs in dogs that have not been vaccinated and it is not seasonal. Symptoms are more severe than the first form, and may include rhinitis, conjunctivitis, and fever in addition to a hacking cough.

Early reports compared the virus to severe acute respiratory syndrome (SARS), and it has been referred to as Saudi Arabia's SARS-like virus. The first person, in June 2012, had a "seven-day history of fever, cough, expectoration, and shortness of breath." One review of 47 laboratory confirmed cases in Saudi Arabia gave the most common presenting symptoms as fever in 98%, cough in 83%, shortness of breath in 72% and myalgia in 32% of people. There were also frequent gastrointestinal symptoms with diarrhea in 26%, vomiting in 21%, abdominal pain in 17% of people. 72% of people required mechanical ventilation. There were also 3.3 males for every female. One study of a hospital-based outbreak of MERS had an estimated incubation period of 5.5 days (95% confidence interval 1.9 to 14.7 days). MERS can range from asymptomatic disease to severe pneumonia leading to acute respiratory distress syndrome (ARDS). Kidney failure, disseminated intravascular coagulation (DIC), and pericarditis have also been reported.

SARS may be "suspected" in a patient who has:

- Any of the symptoms, including a fever of or higher, and

- Either a history of:

1. Contact (sexual or casual) with someone with a diagnosis of SARS within the last 10 days OR

2. Travel to any of the regions identified by the World Health Organization (WHO) as areas with recent local transmission of SARS (affected regions as of 10 May 2003 were parts of China, Hong Kong, Singapore and the town of Geraldton, Ontario, Canada).

For a case to be considered "probable," a chest X-ray must be positive for atypical pneumonia or respiratory distress syndrome.

The World Health Organization (WHO) has added the category of "laboratory confirmed SARS" for patients who would otherwise be considered ""probable"" but who have not yet had a positive chest X-ray changes, but have tested positive for SARS based on one of the approved tests (ELISA, immunofluorescence or PCR).

When it comes to the chest X-ray the appearance of SARS is not always uniform but generally appears as an abnormality with patchy infiltrates.

Those infected with the virus may be asymptomatic or develop flu-like symptoms such as fever, cough, fatigue, and shortness of breath. Emergency symptoms include difficulty breathing, persistent chest pain or pressure, confusion, difficulty waking, and bluish face or lips; immediate medical attention is advised if these symptoms are present. Less commonly, upper respiratory symptoms such as sneezing, runny nose or sore throat may be seen. Gastrointestinal symptoms such as nausea, vomiting and diarrhoea have been observed in varying percentages. Some cases in China initially presented only with chest tightness and palpitations. In some, the disease may progress to pneumonia, multi-organ failure, and death. In those who develop severe symptoms, time from symptom onset to needing mechanical ventilation is typically eight days.

Loss of smell was identified as a common early symptom of COVID-19 in March 2020, although not as common as initially reported.

As is common with infections, there is a delay between the moment when a person is infected with the virus and the time when they develop symptoms. This is called the incubation period. The incubation period for COVID-19 is typically five to six days but may range from two to 14 days. 97.5% of people who develop symptoms will do so within 11.5 days of infection.

Reports indicate that not all who are infected develop symptoms. The role of these asymptomatic carriers in transmission is not yet fully known; however, preliminary evidence suggests that they may contribute to the spread of the disease. The proportion of infected people who do not display symptoms is currently unknown and being studied, with the Korea Centers for Disease Control and Prevention (KCDC) reporting that 20% of all confirmed cases remained asymptomatic during their hospital stay. China's National Health Commission began including asymptomatic cases in its daily cases on 1 April; of the 166 infections on that day, 130 (78%) were asymptomatic at the time of testing.

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The disease was first identified in December 2019 in Wuhan, the capital of China's Hubei province, and has since spread globally, resulting in the ongoing 2019–20 coronavirus pandemic. Common symptoms include fever, cough, and shortness of breath. Other symptoms may include fatigue, muscle pain, diarrhea, sore throat, loss of smell, and abdominal pain. The time from exposure to onset of symptoms is typically around five days but may range from two to fourteen days. While the majority of cases result in mild symptoms, some progress to viral pneumonia and multi-organ failure. As of 17 April 2020, more than 2.23 million cases have been reported across 210 countries and territories, resulting in more than 153,000 deaths. More than 567,000 people have recovered.

The virus is primarily spread between people during close contact, often via small droplets produced by coughing, sneezing, or talking. While these droplets are produced when breathing out, they usually fall to the ground or onto surfaces rather than being infectious over long distances. People may also become infected by touching a contaminated surface and then touching their eyes, nose, or mouth. The virus can survive on surfaces up to 72 hours. It is most contagious during the first three days after the onset of symptoms, although spread may be possible before symptoms appear and in later stages of the disease.

The standard method of diagnosis is by real-time reverse transcription polymerase chain reaction (rRT-PCR) from a nasopharyngeal swab. Chest CT imaging may also be helpful for diagnosis in individuals where there is a high suspicion of infection based on symptoms and risk factors; however, it is not recommended for routine screening.

Recommended measures to prevent infection include frequent hand washing, maintaining physical distance from others (especially from those with symptoms), covering coughs and sneezes with a tissue or inner elbow, and keeping unwashed hands away from the face. The use of masks is recommended for those who suspect they have the virus and their caregivers. Recommendations for mask use by the general public vary, with some authorities recommending against their use, some recommending their use, and others requiring their use. Currently, there is no vaccine or specific antiviral treatment for COVID-19. Management involves treatment of symptoms, supportive care, isolation, and experimental measures.

The World Health Organization (WHO) declared the 2019–20 coronavirus outbreak a Public Health Emergency of International Concern (PHEIC) on 30 January 2020 and a pandemic on 11 March 2020. Local transmission of the disease has been recorded in most countries across all six WHO regions.

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).

Middle East respiratory syndrome (MERS), also known as camel flu, is a viral respiratory infection caused by the MERS-coronavirus (MERS-CoV). Symptoms may range from mild to severe. They include fever, cough, diarrhea, and shortness of breath. Disease is typically more severe in those with other health problems.

MERS-CoV is a betacoronavirus derived from bats. Camels have been shown to have antibodies to MERS-CoV but the exact source of infection in camels has not been identified. Camels are believed to be involved in its spread to humans but it is unclear how. Spread between humans typically requires close contact with an infected person. Its spread is uncommon outside of hospitals. Thus, its risk to the global population is currently deemed to be fairly low.

As of 2016 there is no specific vaccine or treatment for the disease. However, a number of antiviral medications are currently being studied. The World Health Organization recommends that those who come in contact with camels wash their hands frequently and do not touch sick camels. They also recommend that camel products be appropriately cooked. Among those who are infected treatments that help with the symptoms may be given.

Just under 2000 cases have been reported as of April 4, 2017. About 36% of those who are diagnosed with the disease die from it. The overall risk of death may be lower as those with mild symptoms may be undiagnosed. The first identified case occurred in 2012 in Saudi Arabia and most cases have occurred in the Arabian Peninsula. A strain of MERS-CoV known as HCoV-EMC/2012 found in the first infected person in London in 2012 was found to have a 100% match to Egyptian tomb bats. A large outbreak occurred in the Republic of Korea in 2015.

Chicken respiratory diseases are difficult to differentiate and may not be diagnosed based on respiratory signs and lesions. Other diseases such as mycoplasmosis by Mycoplasma gallisepticum (chronic respiratory disease), Newcastle disease by mesogenic strains of Newcastle diseases virus (APMV-1), avian metapneumovirus, infectious laryngotracheitis, avian infectious coryza in some stages may clinically resemble IB. Similar kidney lesions may be caused by different etiologies, including other viruses, such as infectious bursal disease virus (the cause of Gumboro disease) and toxins (for instance ochratoxins of Aspergillus ochraceus), and dehydration.

In laying hens, abnormal and reduced egg production are also observed in Egg Drop Syndrome 76 (EDS), caused by an Atadenovirus and avian metapneumovirus infections. At present, IB is more common and far more spread than EDS. The large genetic and phenotypic diversity of IBV have been resulting in common vaccination failures. In addition, new strains of IBV, not present in commercial vaccines, can cause the disease in IB vaccinated flocks. Attenuated vaccines will revert to virulence by consecutive passage in chickens in densely populated areas, and may reassort with field strains, generating potentially important variants.

Definitive diagnosis relies on viral isolation and characterization. For virus characterization, recent methodology using genomic amplification (PCR) and sequencing of products, will enable very precise description of strains, according to the oligonucleotide primers designed and target gene. Methods for IBV antigens detection may employ labelled antibodies, such as direct immunofluorescence or immunoperoxidase. Antibodies to IBV may be detected by indirect immunofluorescent antibody test, ELISA and Haemagglutination inhibition (haemagglutinating IBV produced after enzymatic treatment by phospholipase C).

The typical symptoms of a cold include a cough, a runny nose, nasal congestion and a sore throat, sometimes accompanied by muscle ache, fatigue, headache, and loss of appetite. A sore throat is present in about 40% of cases and a cough in about 50%, while muscle ache occurs in about half. In adults, a fever is generally not present but it is common in infants and young children. The cough is usually mild compared to that accompanying influenza. While a cough and a fever indicate a higher likelihood of influenza in adults, a great deal of similarity exists between these two conditions. A number of the viruses that cause the common cold may also result in asymptomatic infections.

The color of the sputum or nasal secretion may vary from clear to yellow to green and does not indicate the class of agent causing the infection.

A cold usually begins with fatigue, a feeling of being chilled, sneezing, and a headache, followed in a couple of days by a runny nose and cough. Symptoms may begin within sixteen hours of exposure and typically peak two to four days after onset. They usually resolve in seven to ten days, but some can last for up to three weeks. The average duration of cough is eighteen days and in some cases people develop a post-viral cough which can linger after the infection is gone. In children, the cough lasts for more than ten days in 35–40% of cases and continues for more than 25 days in 10%.

In the United States it is estimated that there are 5 million children with lower respiratory infections (LRI) each year. Estimates have shown that HPIV-1, HPIV-2 and HPIV-3 have been linked with up to a third of these infections. Upper respiratory infections (URI) are also important in the context of HPIV, however are caused to a lesser extent by the virus. The highest rates of serious HPIV illnesses occur among young children and surveys have shown that about 75% of children aged 5 or older have antibodies to HPIV-1.

For infants and young children it has been estimated that ~25% will develop 'clinically significant disease.'

Repeated infection throughout the life of the host is not uncommon and symptoms of later breakouts include upper respiratory tract illness, such as cold and a sore throat. The incubation period for all four serotypes is 1 to 7 days. In immunosuppressed people, parainfluenza virus infections can cause severe pneumonia which can be fatal.

HPIV-1 and HPIV-2 have been demonstrated to be the principal causative agent behind croup (laryngotracheobronchitis) which is a viral disease of the upper airway and is mainly problematic in children aged 6–48 months of age. Biennial epidemics starting in Autumn are associated with both HPIV-1 and 2 however, HPIV-2 can also have yearly outbreaks. Additionally, HPIV-1 tends to cause biennial outbreaks of croup in the fall. In the United States, large peaks have presently been occurring during odd-numbered years.

HPIV-3 has been closely associated with bronchiolitis and pneumonia and principally targets those aged <1 year.

HPIV-4 remains infrequently detected. However, it is now believed to be more common than previously thought, but is less likely to cause severe disease. By the age of 10, the majority of children are sero-positive for HPIV-4 infection which may be indicative of a large proportion of asymptomatic or mild infections.

Important epidemiological factors that are associated with a higher risk of infection and mortality are those who are immuno-compromised and may be taken ill with more extreme forms of LRI. Associations between HPIVs and neurologic disease are known, for example hospitalisation with certain HPIVs has a strong association with febrile seizures. HPIV-4B has the strongest association (up to 62%) followed by hPIV-3 and 1.

HPIVs have also been linked with rare cases of virally caused meningitis and Guillain–Barré syndrome.

HPIVs are spread person to person by contact with infected secretions through respiratory droplets or contaminated surfaces or objects. Infection can occur when infectious material contacts mucous membranes of the eyes, mouth, or nose, and possibly through the inhalation of droplets generated by a sneeze or cough. HPIVs can remain infectious in airborne droplets for over an hour.

Overall, HPIVs remain best known for its effects on the respiratory system and this appears to be where the majority of the focus has been upon.

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).

Symptoms of viral pneumonia include fever, non-productive cough, runny nose, and systemic symptoms (e.g. myalgia, headache). Different viruses cause different symptoms.

Viral pneumonia is a pneumonia caused by a virus.

Viruses are one of the two major causes of pneumonia, the other being bacteria; less common causes are fungi and parasites. Viruses are the most common cause of pneumonia in children, while in adults bacteria are a more common cause.

Feline infectious peritonitis (FIP) is the name given to an uncommon, but usually fatal, aberrant immune response to infection with feline coronavirus (FCoV).

The hallmark clinical sign of effusive FIP is the accumulation of fluid within the abdomen or chest, which can cause breathing difficulties. Other symptoms include lack of appetite, fever, weight loss, jaundice, and diarrhea.

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.

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.

Non-specific effects of vaccines (also called "heterologous effects" or "off-target effects") are effects which go beyond the specific protective effects against the targeted diseases. Non-specific effects can be strongly beneficial, increasing protection against non-targeted infections, but also at times negative, increasing susceptibility to non-targeted infections. This depends on both the vaccine and the sex of the infant.

All live attenuated vaccines studied so far (BCG vaccine, measles vaccine, oral polio vaccine, smallpox vaccine) have been shown to reduce mortality more than can be explained by prevention of the targeted infections. In contrast, inactivated vaccines (diphtheria-tetanus-pertussis vaccine (DTP), hepatitis B vaccine, inactivated polio vaccine) may increase overall mortality despite providing protection against the target diseases.

These effects may be long-lasting, at least up to the time point where a new type of vaccine is given. The non-specific effects can be very pronounced, with significant effects on overall mortality and morbidity. In a situation with herd immunity to the target disease, the non-specific effects can be more important for overall health than the specific vaccine effects.

The non-specific effects should not be confused with the side effects of vaccines (such as local reactions at the side of vaccination or general reactions such as fever, head ache or rash, which usually resolve within days to weeks – or in rare cases anaphylaxis). Rather, non-specific effects represent a form of general immunomodulation, with important consequences for the immune system's ability to handle subsequent challenges.

It is estimated that millions of child deaths in low income countries could be prevented every year if the non-specific effects of vaccines were taken into consideration in immunization programs.

Symptoms arise 4–12 hours after exposure to an organic dust, and generally last from one to five days. Common generalised symptoms include fever over 38 °C, chills, myalgia and malaise. The most frequent respiratory symptoms are dyspnea and a dry cough, while a wheeze may be present less commonly. Headache, rhinitis, conjunctivitis and keratitis can also be present, and skin irritation may occur in those handling grain.

Respiratory function may worsen to the point where hypoxia occurs, and damage to the airways may lead to non-cardiogenic pulmonary edema one to three days post exposure.

Laboratory investigations may show a raised white cell (and specifically neutrophil) count, while a chest X-ray is often normal or shows minimal interstitial infiltration.

Organic dust toxic syndrome (ODTS) is a potentially severe flu-like syndrome originally described in farmers, mushroom workers, bird breeders and other persons occupationally exposed to dusty conditions.

In a typical case, an infant under two years of age develops cough, wheeze, and shortness of breath over one or two days. Crackles or wheeze are typical findings on listening to the chest with a stethoscope. The infant may be breathless for several days. After the acute illness, it is common for the airways to remain sensitive for several weeks, leading to recurrent cough and wheeze.

Some signs of severe disease include:

- poor feeding (less than half of usual fluid intake in preceding 24 hours)

- significantly decreased activity

- history of stopping breathing

- respiratory rate >70/min

- presence of nasal flaring and/or grunting

- severe chest wall recession (Hoover's sign)

- bluish skin