Influenza, a primer – Part I

By Dr M. S. Ramya and Dr. Abhijeeth Chandrasekaran

This article is the first of a 2-part series, and explains the basics of influenza and it’s management. This part (part I) describes the illness, and the next part (part II) will describe the management.

What is flu?

Influenza, or Flu, as it is commonly known is caused by a virus – the influenza virus. There are 3 types of influenza viruses – A, B and C.

A and B viruses cause more serious, seasonal infections, while C virus causes a mild respiratory infection

How do we get flu?

We get flu by being exposed to another person with flu. This could happen via airborne respiratory droplets or via contact with secretions.

When do we get flu?

In countries located at higher latitudes (i.e., closer to the poles), the ‘flu season’ is well defined, and occurs during the colder times of the year. For countries in the Northern Hemisphere, influenza activity often begins to increase in October and November. Most of the time flu activity peaks between December and March and can last as late as May. Corresponding seasons in the Southern Hemisphere are 6 months before or after the season in the Northern Hemisphere.

In a tropical country like India, one can get flu at any time of the year. The virus maintains a low level of infection throughout the year, i.e. small numbers of people keep getting infected with flu throughout the year.

During the flu season however, a large number of people get infected.

Can I get infected with flu even if I have had flu previously?


The nature of influenza virus is such that the viruses keep changing (i.e., mutating), ever so slightly, with time and place. This drift, or change, in the influenza virus is responsible for recurring infections of flu year after year.

What was different with the swine flu pandemic?

Every once in a while, the influenza virus changes more than commonly observed. This happens primarily with the Influenza A type of virus and could be because of interaction with non-human (e.g. pig) flu viruses.

This ‘shift’ results in the influenza virus affecting more individuals than the typical age groups of children and adolescents. If the shift happens such that the virus not just infects people but is also more harmful to health, it has serious consequences and could lead to significant death and disease in all age groups. The last time such a serious infection was observed was during the H1N1 (swine flu) Pandemic of 2009.

What sort of flu can be expected this year?

It is not possible to forecast how flu infections will evolve in a particular season. Though flu occurs every year, the characteristics, such as intensity of the flu season, duration of the flu season change from year to year.

How does influenza affect our body?

Initial symptoms start with fever, severe body ache (myalgia) associated with chills, headache, generalized weakness and loss of appetite (anorexia). Subsequently, you may develop a runny nose (coryza), dry cough and throat pain. Eventually, more generalized body symptoms occur such as abdominal pain, vomiting, and diarrhea.

If untreated, influenza may lead to further complications such as severe lung infection (pneumonia) or ear infection (acute otitis media).

How does the doctor diagnose flu?

Your doctor may depend on different kinds of diagnostic tests to detect flu.

If a large number of people are infected (as may happen in winter), a clinical evaluation (i.e. symptoms, examination) may be adequate to diagnose flu.

Further confirmation of diagnosis is typical, especially during other seasons. This confirmation is through laboratory tests on potentially infective secretions from the nose and throat, with or without other blood tests.

Some of these tests may be quick (e.g. rapid influenza diagnostic test ~30 minutes), while others (e.g. culture 3-10 days) may take longer.

Your doctor will be ideally placed to decide which test is required.

Who is at risk of complicated flu?

Influenza infection can be serious in certain groups of people:

  • Children (especially less than 2 years). As children age, they keep getting exposed to different kinds of influenza viruses and this likely results in immunity against a wide range of different types of influenza viruses.
  • Pregnant women and the immediate post-pregnancy period (till about 2 weeks) – due to changes in the body associated with pregnancy
  • Persons with immunosuppression (caused by medications, HIV infection)
  • Persons with chronic lung, heart, kidney liver or blood diseases or those who are otherwise in ill health (such as morbidly obese individuals)

[Management of influenza to be continued in Part II]

[Dr. M. S. Ramya is a pediatrician, and is currently practicing in Chennai. Dr Abhijeeth Chandrasekaran is a pathologist, and is currently working in the pharmaceutical industry.Corresponding author: Dr. M. S. Ramya, email:]

Disclaimer: The views expressed in this article are solely those of the authors in their private capacity


Nelson textbook of Pediatrics, Twentieth edition. Kliegman R. M, Stanton BF, St. Geme JW, Schor NF, Behrman RE. Elsevier 2016

Influenza (Flu). As provided in the Centers for Disease Control and Prevention (CDC) website accessed at on 7 December 2016.

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