Overview of Respiratory Tract Infection

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I. Definition

RTI is defined as any infectious disease of the upper or lower respiratory tract. Upper respiratory tract infections (URTIs) include the common cold, laryngitis, pharyngitis/tonsillitis, rhinitis, rhinosinusitis/sinusitis and otitis media. Lower respiratory tract infections (LRTIs) include bronchitis, bronchiolitis, pneumonia and tracheitis. The five common respiratory tract infections that are covered by this guideline are: the common cold, pharyngitis/tonsillitis, rhinosinusitis/sinusitis, acute otitis media and acute cough/acute bronchitis.


II. Epidemiology

Respiratory infections often have strong seasonal patterns, with temperate climates more affected during the winter. Several factors explain winter peaks in respiratory infections, including environmental conditions and changes in human behaviors. Viruses that cause respiratory infections are affected by environmental conditions like relative humidity and temperature. Temperate climate winters have lower relative humidity, which is known to increase the transmission of influenza.

Of the viruses that cause respiratory infections in humans, most have seasonal variation in prevalence. Influenza, Human orthopneumovirus (RSV), and human coronaviruses are more prevalent in the winter. Human bocavirus and Human metapneumovirus occur year-round, rhinoviruses (which cause the common cold) occur mostly in the spring and fall, and human parainfluenza viruses have variable peaks depending on the specific strain. Enteroviruses, with the exception of rhinoviruses, tend to peak in the summer.


Amount all the RTIs, the COVID-19 and Influenza are spreading.





Coronaviruses are a large family of viruses that can cause illness in animals or humans. In humans there are several known coronaviruses that cause respiratory infections. These coronaviruses range from the common cold to more severe diseases such as severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and COVID-19.

COVID-19, the respiratory illness responsible for the COVID-19 pandemic, is a contagious disease caused by sever acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which also named as 2019 novel coronavirus (2019-nCoV).



The strain is thought to have been detected by health authorities within weeks of its emergence among the human population in late 2019. It then spread to more than 150 other countries across the world.

The basic reproduction number of the virus has been estimated to be around 5.7. This means each infection from the virus is expected to result in 5.7 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 to reduce the propagation of the virus.

As of 8 January 2021, there have been 88,005,213 total confirmed cases of SARS-CoV-2 infection in the ongoing pandemic. The total number of deaths attributed to the virus is 1,897,568. Many recoveries from both confirmed and untested infections go unreported, since some countries do not collect this data, but at least 49,053,101 people have recovered from confirmed infections. 



COVID-19 affects different people in different ways. Most infected people will develop mild to moderate illness and recover without hospitalization.

Most common symptoms:


dry cough.


Less common symptoms:

aches and pains.

sore throat.




loss of taste or smell.

a rash on skin, or discolouration of fingers or toes.

Serious symptoms:

difficulty breathing or shortness of breath.

chest pain or pressure.

loss of speech or movement.

On average it takes 5–6 days from when someone is infected with the virus for symptoms to show, however it can take up to 14 days.  



A viral test tells you if you have a current infection. Viral test including nucleic acid tests and antigen tests.

An antibody test might tell you if you had a past infection, an important tool for surveillance and epidemiologic studies, such as understanding the transmission dynamic of the virus in the general population. 



Currently, care for patients with COVID-19 is primarily supportive. Care is given to patients to help relieve symptoms and manage respiratory and other organ failure. There are currently no specific antiviral treatments licensed for COVID-19, however many treatments are under investigation. Remdesivir, which is also an investigational drug, received Food and Drug Administration (FDA) emergency use authorization for treatment of hospitalized patients. Finally, a few vaccines are currently available.


IV. Influenza



Influenza (also known as “Flu”) usually refer to a large-scale concept of illness caused by influenza virus infecting both human beings and animals. Seasonal influenza viruses include 4 types, among which the Influenza A and B viruses can cause seasonal epidemics of disease. With its pathogen of high variation and spread capability, flu usually occur outside of the so called “flu season” as people predicted as well.


Influenza A (Flu A) viruses is the only reported type of influenza virus which had caused pandemic worldwide. It has many sub-types classified and recognized according to different combinations of two types of proteins called the hemagglutinin (HA) and the neuraminidase (NA) on the viruses’ surface. Subtype A(H1N1) and A(H3N2) of Flu A are currently circulating in humans.


Unlike Flu A, Influenza B (Flu B) viruses are classified into lineages called B/Yamagata or B/Victoria lineage, taking turn to routinely spread in human beings every year.


Influenza Laboratory Surveillance Information generated on 08/01/2021

Credit: Global Influenza Surveillance and Response System (GISRS)



Influenza is usually considered to be spread via infectious droplets dispersed into the air when patients cough or sneeze to infect persons stay closed, while hands contaminated with influenza viruses as another common access of infection cannot be neglected as well.


Seasonal influenza may mainly occur during winter. However, it may also cause epidemic outbreaks in tropical regions more irregularly.


Despite the whole population can be infected by influenza viruses, there are certain groups that will be at greater risk of severe diseases or complications after being infected including pregnant women, children under 59 months, the elderly, individuals with chronic medical and immunosuppressive conditions. Furthermore, there are groups at higher risk of being infected such as health care workers exposed to the influenza viruses much more frequently than others. Several outbreaks of influenza in human history had caused millions of deaths worldwide.




Estimated Death Worldwide

Flu Type

Sub-type of Flu

Spanish Flu


50 million

Flu A


Asian Flu


1.1 million

Flu A


Hong Kong Flu


1 million

Flu A


H1N1/09 Flu


151~575 thousand

Flu A



Symptoms & Diagnosis

The time from infection to illness (incubation period) is approximately about two days, ranging from one to four days. One may have fever, cough, sore throat, runny or stuffy nose, body aches, headache, chills and/or fatigue. Some people may have vomiting and diarrhea which is more common in children.

However, there’re other viruses causing similar illness as flu, making it impossible for clinicians to confirm if you are infected by flu viruses by symptoms alone. Laboratories have to do tests to tell for sure. There’re currently two mainstream strategies for clinicians to diagnose influenza, which is to detect the specific RNA of the influenza viruses by Nucleic Acid Amplification Technology (NAAT, usually referred as PCR Test) and to detect the antigen of the viruses by Immunochromatography (usually referred as Rapid Test). The pros and cons of these two methods for clinical practice are obvious – the art of triangle balance – price, speed and cost.



NAAT(real time-PCR)

Antigen Test



Lateral Flow

Test Time

1.5-2 hours

15 minutes




Sample Type

Sputum, throat and nasopharyngeal swab

Swab from nose or throat



Less high


Active virus infection

Active virus infection



Hospitals, Companies, Homes…



Antiviral drugs have become the prioritized treatment option for influenza as it can be applied to not only treat the patient’s infection but also controlling the epidemic by preventing severe illness that flu can result including bacterial pneumonia etc. Three antiviral drugs recommended for treating the flu are oseltamivir (Tamiflu®), zanamivir (Relenza®) and peramivir (Rapivab®), which can treat cases of both Flu A and B.



V. Summary


Respiratory pathogens have high potential to initiate a pandemic. Respiratory pathogens can be particularly difficult to contain. Their tendency to have short incubation periods and their potential for asymptomatic spread can mean very small windows are available for interrupting transmission. Individuals infected with respiratory viruses may infect many more people at a time as compared to pathogens spread by other means. These factors increase both the pandemic potential of respiratory pathogens and the likelihood that there will be serious public health, economic, and social impacts with their spread.

That is why Lower respiratory infections remained the world’s most deadly communicable disease, ranked as the 4th leading cause of death. The number of deaths has gone down substantially: in 2019 it claimed 2.6 million lives, 460 000 fewer than in 2000.

Therefore, the best way to prevent the happening of epidemic is to testing early and control at the first time.



[1] https://www.who.int/en/news-room/fact-sheets/detail/influenza-(seasonal)

[2] https://www.cdc.gov/flu

[3] Cécile V, Lone S et al, Global Mortality Impact of the 1957-1959 Influenza Pandemic, PMID: 26908781 PMCID: PMC4747626 DOI: 10.1093/infdis/jiv534 https://pubmed.ncbi.nlm.nih.gov/26908781/

[4] AD Langmuir, SC Schoenbaum., The epidemiology of influenza, PMID: 67988 DOI: 10.1080/21548331.1976.11707011 https://pubmed.ncbi.nlm.nih.gov/67988/

[5]Moriyama, Miyu; Hugentobler, Walter J.; Iwasaki, Akiko (2020). Seasonality of Respiratory Viral Infections

[6] Preparedness for a High-Impact Respiratory Pathogen Pandemic Center for Health Security September 2019


2021-01-08 17:28