Influenza, most commonly referred to as the flu is a contagious viral infection caused primarily by the influenza virus A or B. It generally affects the nose, throat, bronchi, and lungs (i.e. upper respiratory organs) but other organs such as the heart, brain, and muscles can also get affected in severe cases

Other names

  • Flu

Causes Of Influenza

Influenza viruses belong to the family of viruses called Orthomyxoviridae, an RNA-type virus. Viruses have spherical or filamentous shapes with an envelope, containing glycoproteins and a single-stranded RNA gene. The 2 most important glycoproteins over the outer layer of the flu virus are hemagglutinin (H, or HA) and neuraminidase (N, or NA). Both of them have important roles in the spread of the disease.

The influenza viruses are divided into 3 main types i.e A, B, and C. Most of the epidemics (a widespread occurrence of an infectious disease in a community at a particular time) and outbreaks of flu are caused most commonly by types A and B, with type C being generally responsible for sporadic mild upper respiratory symptoms.

Type A influenza virus

For influenza type A, at least 16 highly variable hemagglutinins( a glycoprotein which causes red blood cells to clump together- H1 to H16) and 9 distinct NAs (N1 to N9) have been recognized so far. With the aid of these different antigens, the influenza type A virus is further subdivided into subtypes on the basis of variable combination patterns of their own specific H or N proteins, for example H1N1 or H3N2.

Type B influenza virus

The influenza B virus has a similar viral structure to type A, but, due to the fixed antigenic characters of HA and NA, there are no subtypes in this virus. Still, since the 1970s, some small antigenic variability has been reported in this virus. Also, studies have shown that this virus has started to diverge into 2 antigenically distinguishable lineages.

Symptoms Of Influenza

Each person may experience symptoms in a different manner. Though influenza is a respiratory disease, it can affect the entire body. Sometimes individuals may have trouble figuring out whether they have a common cold or the flu.

There are differences between them like the symptoms of a cold usually come on more slowly and are less severe than symptoms of the flu. Colds rarely cause a fever or headaches. In the case of influenza people usually become very sick with most, or all, of the following symptoms:

  • Cough that becomes severe gradually

  • Extreme exhaustion or tiredness

  • Severe muscle aches and pains

  • Runny or stuffy nose

  • Occasional sneezing

  • Fatigue for several weeks

  • Headache

  • Loss of appetite

  • High fever with chills

  • Sore throat

  • Vomiting and diarrhea (more common in children)

  • Pale face with watery and red eyes

  • Fever and body aches usually last for 3 to 5 days, but cough and fatigue may last for 2 weeks or more. Know the difference between common cold and flu.

Risk factors

  • Adults above the age of 65 years

  • Children below 5 years (especially< 2 years)

  • Pregnant women (2nd or 3rd trimester of pregnancy up to 2 weeks postpartum)

  • Residents of nursing homes and other long-term care facilities

  • Individuals with certain chronic medical conditions like

  • Chronic pulmonary diseases

  • Cardiovascular conditions

  • Diabetes mellitus

  • Kidney disorders

  • Liver diseases


Rapid influenza diagnostic Test (RIDT)

  • Rapid influenza diagnostic tests (RIDTs) are the most common tests used in clinical settings.

  • RIDTs work by detecting the parts of the virus (antigens) that stimulate an immune response.

  • These tests can provide results within approximately 10-15 minutes but may not be as accurate as other flu tests as their reliability depends largely on the conditions under which they are used.

  • Therefore, one could still have influenza, even though your rapid test result is negative.

Rapid molecular assay

  • Rapid molecular assays are a new type of molecular influenza diagnostic test to detect influenza viral RNA or nucleic acids in upper respiratory tract specimens in approximately 15-30 minutes. They are more accurate than RIDTs.

  • In addition to RIDTs and rapid molecular assays, there are more accurate tests available that have to be performed in specialized laboratories.

  • Results may take one to several hours.

  • Proper collection, storage and transport of respiratory specimens is the essential first step for laboratory detection of influenza virus infections.

  • Sample collection requires the healthcare provider to swipe the inside of your nose or the back of your throat with a swab and then send the swab for testing. The tests are discussed below:

Reverse transcription polymerase chain reaction (RT-PCR)

Reverse Transcription-Polymerase Chain Reaction (RT-PCR) can identify the presence of influenza viral RNA in respiratory specimens with very high sensitivity and specificity.

Immunofluorescence assay (direct and indirect)

  • Immunofluorescence assays are antigen detection assays that generally require use of a fluorescent microscope to produce results in approximately 2-4 hours with moderate sensitivity and high specificity.

  • Both direct (DFA) and indirect fluorescent antibody (IFA) staining assays are available to detect influenza A and B viral antigens in respiratory tract specimens.

  • Subtyping or further identification of influenza A viruses is not possible by immunofluorescence assays.

Viral culture (shell vials; cell mixtures)

  • Viral or rapid cell culture results do not yield timely results to inform clinical management.

  • Shell-vial tissue culture results may take 1-3 days, while traditional tissue-cell viral culture results may take 3-10 days. However, viral culture allows for extensive antigenic and genetic characterization of influenza viruses.

  • The collection of some respiratory samples for viral culture is essential for surveillance and antigenic characterization of new seasonal influenza A and B virus strains that may need to be included in next year’s influenza vaccine.

Serologic testing

  • Serological testing for influenza is not recommended for clinical decision-making.

  • Although offered by some commercial laboratories, serological testing results for antibodies to influenza A or B viruses on a single serum specimen cannot be reliably interpreted.

  • Proper serological testing for diagnosis of influenza requires paired acute and convalescent sera collected 2-3 weeks apart, with reliable testing at a limited number of public health or research laboratories to assess a 4-fold or greater rise in influenza virus strain-specific antibodies.

  • Therefore, serological testing for influenza does not provide timely results to help with clinical decisions-making and is not recommended except for research and public health investigations.


Patients with uncomplicated seasonal influenza

Patients that are not from a high risk group should be managed with symptomatic treatment. If they are symptomatic it is best to stay home in order to minimize the risk of infecting others in the community. Treatment mainly focuses on relieving symptoms of influenza such as fever, cough, cold or sore throat. Patients should monitor themselves to detect if their condition deteriorates and seek medical attention.

Patients that are known to be in a group at high risk for developing severe or complicated illness should be treated with antivirals in addition to symptomatic treatment as soon as possible.

Patients with severe or progressive clinical illness

Patients associated with suspected or confirmed influenza virus infection like clinical syndromes of pneumonia, sepsis or exacerbation of chronic underlying diseases, should be treated with antiviral drugs as soon as possible. These drugs may shorten the illness and help prevent serious complications. The drugs are discussed as follows:

Oseltamivir phosphate (Tamiflu)

This is an oral prescription drug used to treat influenza in patients who are two weeks of age and older. It is also approved to prevent flu in patients who are one year of age and older. Important points regarding this drug are:

This drug should be prescribed as soon as possible (ideally, within 48 hours following symptom onset) to maximize therapeutic benefits.

Administration of the drug should also be considered in patients presenting later in the course of illness.

Treatment is recommended for a minimum of 5 days, but can be extended until there is satisfactory clinical improvement.

Potential side effects include nausea, vomiting, nosebleeds, headaches and tiredness.

Zanamivir (Relenza)

This drug is approved to treat flu in patients seven years and older, and to prevent flu in patients who are five and older. This product is inhaled through a device similar to an asthma inhaler. It is not recommended for people who have certain chronic respiratory illnesses like asthma or COPD. Common side effects of the drug include headache, nausea, diarrhea, nose irritation and vomiting.

Peramivir (Rapivab)

Peramivir is approved to treat influenza in patients 2 years and older. This drug is injected into the vein (intravenously) by a healthcare provider. A common side effect of this drug is diarrhea.

Baloxavir (Xofluza)

This drug is approved to treat flu in people 12 years old and older. Common side effects include diarrhea, bronchitis, nausea and headache.

Antiviral drugs

Antiviral drugs like amantadine and rimantadine (Flumadine) are older antiviral drugs which are approved to treat influenza, but most circulating strains of influenza have become resistant to them. Hence, they are no longer recommended.


These should not be used routinely, unless indicated for other reasons (eg: asthma and other specific conditions); as it has been associated with prolonged viral clearance, immunosuppression leading to bacterial or fungal superinfection.

diseases treatments health prevention respiratory-system disorders influenza flu

Subscribe For More Content