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Respiratory Viruses

This guide covers viral respiratory infections, including influenza, RSV, and other influenza-like illnesses (ILI)

What are Influenza-like Illnesses?

Influenza-like Illnesses (ILI)

ILI is a medical diagnosis of possible (or not laboratory-confirmed) influenza or other similar illnesses that cause a common set of symptoms.

Influenza-like illness (ILI) is often defined as fever (>38.0°C) with cough or sore throat, however the broad range of symptoms can include fever, shivering, chills, malaise, dry cough, loss of appetite, body aches, and nausea, and sneezing. These symptoms are typically in connection with a sudden onset of a non-specific respiratory illness and typically resolve on their own within days or weeks.

Common causes of ILI
  • Common Cold (most commonly caused by Rhinoviruses, Coronaviruses [excluding SARS and COVID-19])
  • Suspected [not laboratory confirmed] Influenza
  • Respiratory Syncytial Virus (RSV)
  • Respiratory Adenoviruses
  • Human Parainfluenza Viruses (HPIV)
  • Severe Acute Respiratory Syndrome (SARS)
  • Middle Eastern Respiratory Syndrome (MERS)
  • SARS-CoV-2 (COVID-19)
  • Certain bacterial infections

Common Human Coronaviruses

NOTE: The following surveillance information only applies to the four common human coronavirus types, not SARS-CoV2 or COVID-19.

Coronaviruses are named for the crown-like spikes on their surface. There are four main sub-groupings of coronaviruses, known as alpha, beta, gamma, and delta.

Common human coronaviruses:

  1. 229E (alpha coronavirus)
  2. NL63 (alpha coronavirus)
  3. OC43 (beta coronavirus)
  4. HKU1 (beta coronavirus)

Coronavirus Surveillance

Respiratory Adenovirus

Adenoviruses are medium-sized (90-100 nm), non-enveloped icosohedral viruses with double-stranded DNA. More than 50 types of immunologically distinct adenoviruses can cause infections in humans. Adenoviruses are relatively resistant to common disinfectants and can be detected on surfaces, such as doorknobs, objects, and water of swimming pools and small lakes.

Signs and Symptoms

Adenoviruses most commonly cause respiratory illness. The illnesses can range from the common cold to pneumonia, croup, and bronchitis. Depending on the type, adenoviruses can cause other illnesses such as gastroenteritis, conjunctivitis, cystitis, and, less commonly, neurological disease.

People with weakened immune systems are at high risk for developing severe illness caused by adenovirus infection. Some people infected with adenoviruses, especially those who have weakened immune systems, can have ongoing infections in their tonsils, adenoids, and intestines that do not cause symptoms. They can shed the virus for weeks or longer.

Adenoviruses have historically been a common cause of acute respiratory illness in military recruits, although the frequency has significantly decreased since the reinstitution in March 2011 of adenovirus vaccine administration.


Respiratory Adenovirus Surveillance

Human Parainfluenza Viruses (HPIVs)

Human parainfluenza viruses (HPIVs) belong to the Paramyxoviridae family. They are enveloped RNA viruses. There are four types (1 through 4) and two subtypes (4a and 4b) of HPIVs. The clinical and epidemiological features for each HPIV type can sometimes vary. In the United States, infections associated with HPIV-1 are seen more commonly in odd-numbered years and HPIV-2 and HPIV-3 are seen annually. HPIVs commonly infect infants and young children and persons with weakened immune systems. However, anyone can get HPIV infection.

Symptoms and Illnesses

The incubation period, the time from exposure to HPIV to onset of symptoms, is generally 2 to 7 days.

  • HPIV-1 and HPIV-2 are most often associated with croup. HPIV-1 often causes croup in children, whereas HPIV-2 is less frequently detected. Both types can cause upper and lower respiratory tract illnesses. People with upper respiratory tract illness may have cold-like symptoms.
  • HPIV-3 is more often associated with bronchiolitis, bronchitis, and pneumonia.
  • HPIV-4 is not recognized as often but may cause mild to severe respiratory tract illnesses.


People can get multiple HPIV infections in their lifetime. These reinfections usually cause mild upper respiratory tract illness with cold-like symptoms. However, reinfections can cause serious lower respiratory tract illness, such as pneumonia, bronchitis, and bronchiolitis in some people. Older adults and people with compromised immune systems have a higher risk for severe infections.

Most children 5 years of age and older have antibodies against HPIV-3 and approximately 75% have antibodies against HPIV-1 and HPIV-2.


HPIVs usually spread by direct contact with infectious droplets or by airborne spread when an infected person breathes, coughs, or sneezes.

HPIVs may remain infectious in airborne droplets for over an hour and on surfaces for a few hours depending on environmental conditions.

People are most contagious during the early stage of illness.

People usually get HPIV infection in the spring, summer, and fall. However, it is possible to get infected at any time of the year. For more information, see HPIV Seasons.

Prevention and Treatment

Currently, there is no vaccine to prevent HPIV infection. However, researchers are trying to develop vaccines. Also, there is no specific antiviral treatment for HPIV illness. Most HPIV illnesses are mild and typically require only treatment of symptoms.

In hospital settings, healthcare providers should follow contact precautions, such as handwashing and wearing protective gowns and gloves.


Human Parainfluenza Virus Surveillance

Human Metapneumonovirus (HMPV)

Human metapneumovirus (HMPV) can cause upper and lower respiratory disease in people of all ages, especially among young children, older adults, and people with weakened immune systems. Discovered in 2001, HMPV is in the paramyxovirus family along with respiratory syncytial virus (RSV). Broader use of molecular diagnostic testing has increased identification and awareness of HMPV as an important cause of upper and lower respiratory infection.


Symptoms commonly associated with HMPV include cough, fever, nasal congestion, and shortness of breath. Clinical symptoms of HMPV infection may progress to bronchitis or pneumonia and are similar to other viruses that cause upper and lower respiratory infections. The estimated incubation period is 3 to 6 days, and the median duration of illness can vary depending upon severity but is similar to other respiratory infections caused by viruses.


HMPV is most likely spread from an infected person to others through

  • secretions from coughing and sneezing
  • close personal contact, such as touching or shaking hands
  • touching objects or surfaces that have the viruses on them then touching the mouth, nose, or eyes

In the U.S., HMPV circulates in distinct annual seasons. HMPV circulation begins in winter and lasts until or through spring. HMPV, RSV, and influenza can circulate simultaneously during the respiratory virus season.


Human Metapneumovirus Surveillance